Update RELEASES.md

Co-authored-by: Jubilee <46493976+workingjubilee@users.noreply.github.com>

RELEASES.md

@@ -142,6 +142,7 @@ Compatibility Notes
 - [Make `where_clause_object_safety` lint a regular object safety violation](https://github.com/rust-lang/rust/pull/125380/)
 - [Turn `proc_macro_back_compat` lint into a hard error.](https://github.com/rust-lang/rust/pull/125596/)
 - [Detect unused structs even when implementing private traits](https://github.com/rust-lang/rust/pull/122382/)
+- [`std::sync::ReentrantLockGuard<T>` is no longer `Sync` if `T: !Sync`](https://github.com/rust-lang/rust/pull/125527) which means [`std::io::StdoutLock` and `std::io::StderrLock` are no longer Sync](https://github.com/rust-lang/rust/issues/127340)
 
 <a id="1.80-Internal-Changes"></a>