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/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)