Cleanup release content (one more time) (#24198)

# Objective

- Migration guide merged in release-content

## Solution

- Move it
- Add a CI check that will block new merges

Opened PRs that sill change that folder:
- #23467
- #23445
- #23373
- #23137
- #23132
- #23056
- #22917
- #22852
- #22782
- #22670
- #22557
- #22500
- #21929
- #21912
- #21897
- #21893
- #21890
- #21889
- #21839
- #21811
- #21772

None of them are likely to be merged in the 0.19