README: Discontinue Ubuntu packages

Reasons:

- Low user numbers, according to Launchpad API download counts.
  - As of 2024-12-16 for latest published packages
    since Sep 2024 (4 months):
    - 20.04: 2
    - 22.04: 8
    - 24.04: 3
  - There were highs of 71 for 20.04 and 227 for 22.04 for previous
    builds. Unless most people deleted the repo afterwards, or there
    was a lot of reinstalls during that time, suggests not many people
    kept using the repository for updates.

- High effort, lack of motivation.
- Medium risk due to the complexity of this kind of packaging.

In the long term, maintaining these Ubuntu packages is not something
I'd like to do into 2025 and beyond. Happy to pass this on to someone
else keen to maintain and test Ubuntu packages.

From this point, this project only guarantees packages for Arch Linux.

README.md

@@ -37,7 +37,7 @@ While the patched installation should just work, you are entering unsupported te
 
 * [Arch Linux](#arch-linux)
 * [Gentoo](#gentoo)
-* [Ubuntu (LTS)](#ubuntu)
+* [Ubuntu (LTS)](#ubuntu) (discontinued)
 
 ### Arch Linux
 
@@ -80,9 +80,15 @@ built autonomously by [GitHub Actions](https://github.com/lah7/gtk3-classic/acti
 
 ### Ubuntu
 
-[![Ubuntu](https://img.shields.io/badge/Ubuntu-ppa%3Alah7%2Fgtk3--classic-e95420?logo=ubuntu&logoColor=white)](https://launchpad.net/~lah7/+archive/ubuntu/gtk3-classic)
+> [!WARNING]
+> Ubuntu packages for gtk3-classic has been discontinued.
+> The existing PPA will continue to work, but no new packages will be created.
+> **Use at your own risk.**
+>
+> [You may build them yourself](https://github.com/lah7/gtk3-classic/wiki/Ubuntu-Packaging) or contribute improvements to reliably build locally via our [create-ubuntu-package.sh](scripts/create-ubuntu-package.sh) script.
 
-> **Only available for LTS releases** (starting from 20.04)
+> [!NOTE]
+> **Only available for LTS releases:** 20.04, 22.04, 24.04
 
 These packages are based on the original [`gtk+3.0`](https://packages.ubuntu.com/search?section=all&arch=any&keywords=gtk%2B3.0&searchon=sourcenames)
 package, with the version suffixed with `~classic`. These packages are rarely