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/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