Bump the package version to 2.3.0 (setup.cfg, docs/conf.py) and add the
changelog entry for the release.

This is the free-threading release. On top of 2.2.0 the extension is now
declared free-threading compatible, so importing it on a free-threaded
("nogil") CPython 3.13t/3.14t no longer re-enables the GIL, and a single DB is
safe to share across threads: the data path stays lock-free, while open/close
and column-family management are serialized per DB by a reentrant lock, the C++
callback trampolines hold a counted reference to their Python context, and the
Options / column-family-handle check-then-use races were made atomic. Closing a
DB with a live iterator or snapshot no longer aborts/segfaults. CI gained a
free-threaded 3.14t leg and a ThreadSanitizer leg.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>