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/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
 ```