Add a missing compatibility note in the 1.80.0 release notes

Author: apiraino

RELEASES.md

@@ -143,6 +143,7 @@ Compatibility Notes
 - [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)
+- [alloc: implement FromIterator for Box<str>](https://github.com/rust-lang/rust/pull/99969/)
 
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