OS support spring cleaning spree

[Jackenmen]

Description of the changes

Time to do some cleaning!

Changes:

• Added Windows 10's retirement date now that it is set in stone
• Removed support for CentOS Stream 8, Fedora 38, RHEL 8.6, and RHEL 9.0 which all went EOL by the end of May 2024
• Removed support for CentOS 7 which went EOL after June 30
• Removed all mentions of pyenv now that the last OS using it (CentOS 7) is no longer supported
• Bumped Python version on RHEL 8 and 9 derivatives to 3.11 now that we no longer support RHEL 8.6 and 9.0
• Bumped Python version on Arch Linux to 3.11 now that its default Python is 3.12

Docs link: https://red-discordbot--6386.org.readthedocs.build/en/6386/install_guides/index.html

Have the changes in this PR been tested?

Yes

macOS 12 & 13 x86_64: https://github.com/Jackenmen/Red-Install-Tests/actions/runs/9733598090
Everything else: https://cirrus-ci.com/build/6177043149225984

[Flame442]

Debian 11 is also supposed to EOL this July, but they still haven't given an exact date despite it being July tomorrow. If the release takes long enough, that's something to consider. Maybe wait to merge this until release to confirm that?

[Jackenmen]

Maybe wait to merge this until release to confirm that?

I'll make a separate PR once Debian 11 actually reaches EOL, I don't want to block the merging of this on that.