docs: rust: quick-start: openSUSE provides `rust-src` package nowadays
Both openSUSE Tumbleweed and Slowroll provide the `rust-src` package
nowadays [1].
Thus remove the version-specific one from the Quick Start guide.
Link: https://software.opensuse.org/package/rust-src?search_term=rust-src [1]
Reviewed-by: Tamir Duberstein <tamird@kernel.org>
Link: https://patch.msgid.link/20260405235309.418950-23-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>