docs: release notes for v0.21.4 (replaces never-shipped v0.21.3 draft)

v0.21.3 was drafted but never tagged — its commits flow into v0.21.4
along with the issue #85 fix, the diagnostics redaction fix, and the
compaction-markers always-on graduation.

The v0.21.3 draft is replaced by v0.21.4.md which leads with
compaction-markers graduation and adds the two new fixes.