chore: remove references to ubuntu 18.04 ahead of EOL

While we will still build for 18.04, we will now stop testing against it. Instead, we will test against 20.04, the next EOL release after 18.04.

Once Ubuntu 18.04 is EOL, any references to it will also be removed.

Note that 18.04 support was already deprecated in Dokku 0.28.0, so this change is just acknowledging that deprecation.

docs/getting-started/install/debian.md

@@ -12,8 +12,8 @@ wget -nv -O - https://get.docker.com/ | sh
 
 # install dokku
 wget -qO- https://packagecloud.io/dokku/dokku/gpgkey | sudo tee /etc/apt/trusted.gpg.d/dokku.asc
-OS_ID="$(lsb_release -cs 2>/dev/null || echo "bionic")"
-echo "bionic focal jammy" | grep -q "$OS_ID" || OS_ID="bionic"
+OS_ID="$(lsb_release -cs 2>/dev/null || echo "focal")"
+echo "focal jammy" | grep -q "$OS_ID" || OS_ID="focal"
 echo "deb https://packagecloud.io/dokku/dokku/ubuntu/ ${OS_ID} main" | sudo tee /etc/apt/sources.list.d/dokku.list
 sudo apt-get update -qq >/dev/null
 sudo apt-get -qq -y install dokku

docs/getting-started/install/dreamhost.md

@@ -19,7 +19,7 @@ source openrc.sh # Set the environment variables for DreamHost Cloud
 
 This allows OpenStack client to connect to DreamHost API endpoints.
 The command below creates a new server instance named `my-dokku-instance`
-based on Ubuntu 18.04, with 2 GB RAM and 1 CPU (the flavor called
+based on Ubuntu 20.04, with 2 GB RAM and 1 CPU (the flavor called
 `supersonic`), opening network port access to HTTP and SSH (the
 `default` security group), and the name of the chosen SSH key. This
 key will be automatically added to the new server in the
@@ -28,7 +28,7 @@ be reused by Dokku.
 
 ```sh
 openstack server create \
-  --image Ubuntu-18.04 \
+  --image Ubuntu-20.04 \
   --flavor gp1.supersonic \
   --security-group default \
   --key-name $YOUR_SSH_KEYNAME \

docs/getting-started/installation/index.md

@@ -9,7 +9,7 @@ Dokku is an extensible, open source Platform as a Service that runs on a single
 To start using Dokku, you'll need a system that meets the following minimum requirements:
 
 - A fresh installation of any of the following operating systems:
-    - [Ubuntu 18.04/20.04/22.04](https://www.ubuntu.com/download)
+    - [Ubuntu 20.04/22.04](https://www.ubuntu.com/download)
     - [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