rust: conclude the Rust experiment

The Rust support was merged in v6.1 into mainline in order to help
determine whether Rust as a language was suitable for the kernel,
i.e. worth the tradeoffs, technically, procedurally and socially.

At the 2025 Linux Kernel Maintainers Summit, the experiment has just
been deemed concluded [1].

Thus remove the section -- it was not fully true already anyway, since
there are already uses of Rust in production out there, some well-known
Linux distributions enable it and it is already in millions of devices
via Android.

Obviously, this does not mean that everything works for every kernel
configuration, architecture, toolchain etc., or that there won't be
new issues. There is still a ton of work to do in all areas, from the
kernel to upstream Rust, GCC and other projects. And, in fact, certain
combinations (such as the mixed GCC+LLVM builds and the upcoming GCC
support) are still quite experimental but getting there.

But the experiment is done, i.e. Rust is here to stay.

I hope this signals commitment from the kernel to companies and other
entities to invest more into it, e.g. into giving time to their kernel
developers to train themselves in Rust.

Thanks to the many kernel maintainers that gave the project their
support and patience throughout these years, and to the many other
developers, whether in the kernel or in other projects, that have
made this possible. I had a long list of 173 names in the credits of
the original pull that merged the support into the kernel [2], and now
such a list would be way longer, so I will not even try to compose one,
but again, thanks a lot, everybody.

Link: https://lwn.net/Articles/1050174/ [1]
Link: https://git.kernel.org/linus/8aebac82933f [2]
Reviewed-by: Gary Guo <gary@garyguo.net>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Benno Lossin <lossin@kernel.org>
Acked-by: Andreas Hindborg <a.hindborg@kernel.org>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://patch.msgid.link/20251213000042.23072-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Documentation/process/programming-language.rst

@@ -34,7 +34,7 @@ Please refer to ``include/linux/compiler_attributes.h`` for more information.
 Rust
 ----
 
-The kernel has experimental support for the Rust programming language
+The kernel has support for the Rust programming language
 [rust-language]_ under ``CONFIG_RUST``. It is compiled with ``rustc`` [rustc]_
 under ``--edition=2021`` [rust-editions]_. Editions are a way to introduce
 small changes to the language that are not backwards compatible.