Bump the package version to 2.1.0 (setup.cfg, docs/conf.py) and finalize
the changelog for the release.

On top of 2.0.0 this release collects:
- the iterskeys/itersitems column-family shadowing fix, a use-after-close
  RuntimeError guard (in place of a segfault), and a TransactionDB
  double-close fix; and
- a lowered RocksDB floor (8.x, down from 9) with a fail-fast build guard
  that aborts with a clear message on an unsupported RocksDB.

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