Python-only patch release: librdkafka stays at v2.16.0.

- Bump version to 2.16.1rc1 in pyproject.toml, CFL_VERSION_STR and the
  soak test version list
- CHANGELOG: move the MetadataTags (#2385) and Windows TopicPartition
  (#2374) fixes into v2.16.1, and note the bundled librdkafka version

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>