Add 4.21.4 changelog

Editor assessment

The subject correctly identifies the 4.21.4 changelog addition, which is complete for this kind of mechanical docs commit. The body then degenerates into bullets reading `fix` and `reset` with no objects, which are noise rather than explanation. The docs type fits and no scope is needed. No footer is present. Hygiene is marred by those empty bullets, though the subject itself is clean and free of junk. The message is valid and understandable, but its contents teach a reader nothing beyond the version number.