rust: kernel: update file_with_nul comment

`feature(file_with_nul)` [1] has been stabilized in Rust 1.92.0 [2].

Thus update the comment to keep track of it.

In addition, this will help to sort new conditionally enabled features
(i.e. `cfg_attr`) around it appropriately.

Link: rust-lang/rust#141727 [1]
Link: rust-lang/rust#145664 [2]
Link: https://patch.msgid.link/20260406095820.465994-1-ojeda@kernel.org
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Boqun Feng <boqun@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

rust/kernel/lib.rs

@@ -23,7 +23,7 @@
 // To be determined.
 #![feature(used_with_arg)]
 //
-// `feature(file_with_nul)` is expected to become stable. Before Rust 1.89.0, it did not exist, so
+// `feature(file_with_nul)` is stable since Rust 1.92.0. Before Rust 1.89.0, it did not exist, so
 // enable it conditionally.
 #![cfg_attr(CONFIG_RUSTC_HAS_FILE_WITH_NUL, feature(file_with_nul))]