Fix codecov action and update whats_new.rst

[SpacemanPaul]

Reason for this pull request

Codecov github action was broken by #1542. This PR fixes it.

Note that this change is applied for develop-1.9 in #1553

Proposed changes

• Pass Codecov auth token as repo secret, as required by codecov-action@v4

• Tests added / passed

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

---

📚 Documentation preview 📚: https://datacube-core--1554.org.readthedocs.build/en/1554/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3243808) 91.77% compared to head (959207b) 91.77%.
Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1554   +/-   ##
========================================
  Coverage    91.77%   91.77%           
========================================
  Files          132      132           
  Lines        14631    14631           
========================================
  Hits         13428    13428           
  Misses        1203     1203           

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[omad]

Thanks!