docs: rust: quick-start: add Ubuntu 26.04 LTS and remove subsection title

Ubuntu 26.04 LTS (Resolute Raccoon) is scheduled to be released in a few
weeks [1], and it has a recent enough Rust toolchain, just like Ubuntu
25.10 has [2][3].

We could update the title and the paragraph, but to simplify and to
make it more consistent with the other distributions' sections, let's
instead just remove that title. It will also reduce the differences
later on to keep it updated. Eventually, when we remove the remaining
subsection for older LTSs, Ubuntu should be a small section like the
other distributions.

Thus remove the title and add the mention of Ubuntu 26.04 LTS.

Link: https://documentation.ubuntu.com/release-notes/26.04/schedule/#resolute-raccoon-schedule [1]
Link: https://packages.ubuntu.com/search?keywords=rustc&searchon=names&exact=1&suite=all&section=all [2]
Link: https://packages.ubuntu.com/search?keywords=bindgen&searchon=names&exact=1&suite=all&section=all [3]
Reviewed-by: Tamir Duberstein <tamird@kernel.org>
Link: https://patch.msgid.link/20260405235309.418950-26-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>