docs: rust: quick-start: update minimum Ubuntu version

Ubuntu 25.04 is out of support [1], and Ubuntu 25.10 is the latest
supported one.

Moreover, Ubuntu 25.10 is the first that provides a recent enough Rust
given the minimum bump -- they provide 1.85.1 [2].

Thus update it.

Link: https://ubuntu.com/about/release-cycle [1]
Link: https://packages.ubuntu.com/search?keywords=rustc&searchon=names&exact=1&suite=all&section=all [2]
Reviewed-by: Tamir Duberstein <tamird@kernel.org>
Link: https://patch.msgid.link/20260405235309.418950-25-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>