Bump the package version to 2.2.0 (setup.cfg, docs/conf.py) and finalize the
changelog for the release.

On top of 2.1.0 this release hardens the build's RocksDB version gate: a major
newer than the tested range (currently 10.x) is now refused by both setup.py
and the version_check.hpp compile-time backstop, where 2.1.0 only warned.
Building against an untested RocksDB major is risky, so the supported range is
now enforced on both ends.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>