Update whats_new.rst ready for 1.9.0-rc6 pre-release.

[SpacemanPaul]

Reason for this pull request

Need another pre-release to address bugs identified during 1.9 migration of other ODC packages.

Proposed changes

• Update whats_new.rst ready for release.

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.43%. Comparing base (7bda871) to head (e0dbe45).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop-1.9 #1598 +/- ## ============================================ Coverage 85.43% 85.43% ============================================ Files 140 140 Lines 15496 15496 ============================================ Hits 13239 13239 Misses 2257 2257 ```

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