Update whats_new.rst for 1.8.19 release

[SpacemanPaul]

Reason for this pull request

Time for a 1.8 release

Proposed changes

• Update whats_new.rst for release

  • [x] Tests added / passed
  • [x] Fully documented, including docs/about/whats_new.rst for all changes

---

📚 Documentation preview 📚: https://datacube-core--1612.org.readthedocs.build/en/1612/