Rust version policy: add Ubuntu to the list

Ubuntu now appears to provide the versioned packages in all LTS (at least
the ones with standard support) and the latest non-LTS (interim) release.

Link: https://lore.kernel.org/rust-for-linux/20240816181545.45789-1-ojeda@kernel.org/
Link: https://packages.ubuntu.com/search?keywords=rustc-1
Link: https://packages.ubuntu.com/search?keywords=bindgen-0
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

src/Rust-version-policy.md

@@ -25,6 +25,7 @@ The Rust versions currently supported should already be enough for kernel develo
   - Gentoo Linux.
   - Nix (unstable).
   - openSUSE Slowroll and Tumbleweed.
+  - Ubuntu LTS (20.04, 22.04, 24.04) and non-LTS (interim).
 
 In addition, we support the toolchains distributed by Rust, installed via [`rustup`](https://rust-lang.github.io/rustup/) or the [standalone installers](https://forge.rust-lang.org/infra/other-installation-methods.html#standalone-installers).