rust: kbuild: update `bindgen --rust-target` version and replace comment

rust: kbuild: update `bindgen --rust-target` version and replace comment

As the comment in the `Makefile` explains, previously, we needed to
limit ourselves to the list of Rust versions known by `bindgen` for its
`--rust-target` option [1].

In other words, we needed to consult the versions known by the minimum
version of `bindgen` that we supported.

Now that we bumped the minimum version of `bindgen`, that limitation
does not apply anymore since `bindgen` 0.71.0 [2].

Thus replace the comment and simply write our minimum supported Rust
version there, which is much simpler.

See commit 7a5f93ea5862 ("rust: kbuild: set `bindgen`'s Rust target
version") for more details.

Link: https://rust-lang.zulipchat.com/#narrow/channel/425075-rust-for-linux/topic/rust.20version.20on.20generated.20bindings/near/484087179
Link: https://github.com/rust-lang/rust-bindgen/pull/2993
Reviewed-by: Tamir Duberstein <tamird@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260405235309.418950-21-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>