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/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)