release: v8.4.0-DMR

### Disclaimer
The dates published in this issue are for planning purposes only. It is intended solely to help you plan your projects. A date that exists in the future should not be considered a firm release date as the release and timing of releases are subject to change at any time and are at PingCAP's sole discretion.

### Release process checklist

### Candidate SHA:
TBD

Working Tags: TBD

### Check the diff in the current version:

- [x] 0. Announce the release plan
- [ ] 1. Create release-8.4 branch: Sep. 26, 2024
- [ ] 2. Code freeze: Sep. 26, 2024
- [ ] 3. Qualifying the release.
- [ ] 4. Docs preparing for release.
- [ ] 5. The release qualified.
- [ ] 6. Docs ready for release.

**release date: Oct. 2024**

Do not proceed below until the release date.

- [ ] 7. Publish TiDB Release
> - [ ] fill in Candidate SHA above
> - [ ] fill in Make and Publish Build above
- [ ] 8. Update version numbers
- [ ] 9. Update docs (handled by Docs team)
- [ ] 10. Update the official website