docs: rust: quick-start: remove Gentoo "testing" note
Gentoo does not need the "testing" note, since its packages are recent
enough even in the stable branch [1][2].
Thus remove it to simplify the documentation.
Link: https://packages.gentoo.org/packages/dev-lang/rust [1]
Link: https://packages.gentoo.org/packages/dev-util/bindgen [2]
Reviewed-by: Tamir Duberstein <tamird@kernel.org>
Link: https://patch.msgid.link/20260405235309.418950-27-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>