Update whats_new.rst for 1.8.18 release.

[SpacemanPaul]

Reason for this pull request

Administrivia ready for 1.8.18 release

Proposed changes

• Update whats_new.rst.

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

---

📚 Documentation preview 📚: https://datacube-core--1568.org.readthedocs.build/en/1568/

[codecov[bot]]

Codecov Report

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

Project coverage is 91.79%. Comparing base (fa58d90) to head (5f3fef3).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1568 +/- ## ======================================== Coverage 91.79% 91.79% ======================================== Files 132 132 Lines 14633 14633 ======================================== Hits 13432 13432 Misses 1201 1201 ```

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