chore(release): v4.3.0-rc.1

- Prepend v4.3.0-rc.1 entry to CHANGELOG.md (162 entries grouped into
  Breaking / Features / Bug Fixes / Documentation / Miscellaneous).
- Add release-notes-4.3.0-rc.1.md narrative for the GitHub release body.
- Rename '## TBD' to '## 4.3.0-rc.1' in migration_notes.md; '## TBD'
  remains empty for the next release cycle.

The CHANGELOG entry is auto-generated and uncurated. The narrative
release-notes file groups the most significant changes; curate before
publishing.

Author: aminsammara