Documentation: Add list of supported Ubuntu / Debian releases to README.md

[realHenningLorenzen]

I could not install deb-get on the development version of Ubuntu 25.04.

  [!] ERROR! Ubuntu Plucky is not supported because it is not derived from a supported Debian or Ubuntu release.

I guess this is my mistake, but by that occasion I tried to look up which releases are supported and was having a hard time doing that.

[rasa]

@realHenningLorenzen plucky support was added on Jan 18, 2025, but deb-get hasn't had a release since then. Perhaps it's time. @philclifford ?

Note that the documentation mentions it in the EXTREPO.md doc file, which says:

UPSTREAM_CODENAME: The codename of the upstream distribution. Supported values are buster (10), bullseye (11), bookworm (12), trixie (13), sid (unstable), focal (20.04), jammy (22.04), lunar (23.04), mantic (23.10) and noble (24.04)

But that statement is outdated. As of now, 21 packages support oracular (24.10) and five packages support plucky (25.04).

Adding support for a release is done manually, and not done all at once for all packages, but individually, when a package adds support for that release. And we generally aren't checking each package's releases to see if it's added support for the OS's latest release. @philclifford may have more input on this.

[philclifford]

Definitely time for a release - ideally before plucky is released. (I lack the privs but hopefully @flexiondotorg can merge some of my pending PRs and a couple that are working but stuck, and trigger a release).