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>

Author: ojeda

Documentation/rust/quick-start.rst

@@ -57,8 +57,8 @@ of the box, e.g.::
 Gentoo Linux
 ************
 
-Gentoo Linux (and especially the testing branch) provides recent Rust releases
-and thus it should generally work out of the box, e.g.::
+Gentoo Linux provides recent Rust releases and thus it should generally work out
+of the box, e.g.::
 
 	USE='rust-src rustfmt clippy' emerge dev-lang/rust dev-util/bindgen