doc: Update Changelog

Add entry about the UFFD fix.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

Author: roypat

CHANGELOG.md

@@ -26,6 +26,9 @@ and this project adheres to
 - [#4926](https://github.com/firecracker-microvm/firecracker/pull/4926): Replace
   underlying implementation for seccompiler from in house one in favor of
   `libseccomp` which produces smaller and more optimized BPF code.
+- [#5021](https://github.com/firecracker-microvm/firecracker/pull/5021) If a
+  balloon device is inflated post UFFD-backed snapshot restore, Firecracker now
+  causes `remove` UFFD messages to be sent to the UFFD handler.
 
 ### Deprecated