chore: drop unsupported operating systems

We will no longer support CentOS, Fedora, and Opensuse as installation targets. These are not actively maintained by anyone with commit rights and occasionally cause issues for users as they are not tested during the release process.

Rather than have subpar support for an untested operating system, we're removing support for them completely. Users of these operating systems should take a look migration to the docker-based installation method, which will always be tested and supported by the project.

Additionally, drop support for Debian 9 as it is now EOL.

docs/appendices/0.28.0-migration-guide.md

@@ -0,0 +1,18 @@
+# 0.28.0 Migration Guide
+
+## Removals
+
+Support for the following operating systems has been removed:
+
+- CentOS 7
+- Debian 9 (Stretch)
+- Fedora
+- OpenSuse
+
+Dokku will no longer provide packages for RPM based systems. Debian 9 users _may_ use new Debian packages, though support is not guaranteed.
+
+All users are encouraged to install Dokku via the [Docker-based installation method](/docs/getting-started/install/docker.md), or switch to a supported operating system.
+
+## Deprecations
+
+Ubuntu 18.04 is now a deprecated installation target. The operating system will be considered EOL by Canonical in April 2023. Users are encouraged to upgrade to Ubuntu 22.04 or consider switching their instllation method to the [Docker-based installation method](/docs/getting-started/install/docker.md) to avoid any disruption in usage.

docs/development/plugin-triggers.md

@@ -441,7 +441,6 @@ DOKKU_SCHEDULER="$1"
 ```shell
 #!/usr/bin/env bash
 # Installs nginx for the current plugin
-# Supports both opensuse and ubuntu
 
 set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
 
@@ -451,9 +450,8 @@ case "$DOKKU_DISTRO" in
   debian|raspbian|ubuntu)
     apt-get -qq -y --no-install-recommends install nginx
     ;;
-
-  opensuse)
-    zypper -q in -y nginx
+  *)
+    echo "Installation on $DOKKU_DISTRO not supported" 1>&2
     ;;
 esac
 ```

docs/getting-started/advanced-installation.md

@@ -12,7 +12,6 @@ As well, you may wish to customize your installation in some other fashion. or e
 
 - [Debian Package Installation Notes](/docs/getting-started/install/debian.md)
 - [Docker-based Installation Notes](/docs/getting-started/install/docker.md)
-- [RPM Package Installation Notes](/docs/getting-started/install/rpm.md)
 - [Vagrant Installation Notes](/docs/getting-started/install/vagrant.md)
 - [Advanced Install Customization](/docs/getting-started/advanced-installation.md)
 - [Automated deployment via ansible](https://github.com/dokku/ansible-dokku)

docs/getting-started/install/rpm.md

@@ -1,27 +0,0 @@
-# RPM Package Installation Notes
-
-> New as of 0.8.0 *(experimental)*
-
-> **Warning:** Web installer is not available on CentOS. You will need to configure [SSH keys](/docs/deployment/user-management.md#adding-ssh-keys) and [virtual hosts](/docs/configuration/domains.md#customizing-hostnames) using dokku command line interface.
-
-Dokku defaults to being installed via RPM package on CentOS 7. While certain hosts may require extra work to get running, you may optionally wish to automate the installation of Dokku without the use of our `bootstrap.sh` Bash script. The following are the steps run by said script:
-
-```shell
-# Install docker
-curl -fsSL https://get.docker.com/ | sh
-
-# Install epel for nginx packages to be available
-sudo yum install -y epel-release
-
-# Install dokku
-curl -s https://packagecloud.io/install/repositories/dokku/dokku/script.rpm.sh | sudo bash
-sudo yum install -y herokuish dokku
-sudo dokku plugin:install-dependencies --core
-systemctl start nginx
-
-# Enable docker and nginx on system startup
-systemctl enable docker
-systemctl enable nginx
-```
-
-The [devicemapper](https://docs.docker.com/engine/userguide/storagedriver/device-mapper-driver/) is the default Docker storage driver on CentOS 7 and defaults to a configuration mode known as `loop-lvm`. This mode is designed to work out-of-the-box with no additional configuration. However, production deployments should not run under `loop-lvm` mode. The preferred configuration for production deployments is [`direct-lvm`](https://docs.docker.com/engine/userguide/storagedriver/device-mapper-driver/#/configure-direct-lvm-mode-for-production).

docs/getting-started/installation.md

@@ -10,8 +10,7 @@ To start using Dokku, you'll need a system that meets the following minimum requ
 
 - A fresh installation of any of the following operating systems:
     - [Ubuntu 18.04/20.04/22.04](https://www.ubuntu.com/download)
-    - [Debian 9+ x64](https://www.debian.org/distrib/)
-    - [CentOS 7 x64](https://www.centos.org/download/) *(experimental)*
+    - [Debian 10+ x64](https://www.debian.org/distrib/)
 - A server with one of the following architectures
     - AMD64 (alternatively known as `x86_64`), commonly used for Intel cloud servers 
     - ARMV7 (alternatively known as `armhf`), commonly used for Raspberry PI

docs/getting-started/uninstalling.md

@@ -9,13 +9,6 @@ While we hate to see you go, if you need to uninstall Dokku, the following may h
 yay -Rsn dokku
 ```
 
-## CentOS Uninstallation
-
-```shell
-# uninstall dokku
-yum remove dokku herokuish
-```
-
 ## Debian Uninstallation
 
 ```shell

docs/getting-started/upgrading.md

@@ -9,7 +9,6 @@ For any security related updates, please follow our [Twitter account](https://tw
 Your operating system may occasionally provide security updates. We recommend setting unattended upgrades for your operating system. Here are some helpful links:
 
 - [Arch Linux System Maintenance](https://wiki.archlinux.org/index.php/System_maintenance)
-- [CentOS Automatic Security Updates](https://serversforhackers.com/c/automatic-security-updates-centos)
 - [Debian Unattended Upgrades](https://wiki.debian.org/UnattendedUpgrades)
 - [Ubuntu Unattended Upgrades](https://help.ubuntu.com/community/AutomaticSecurityUpdates)