Update 1.8 release process for post 1.9.0 release.

[SpacemanPaul]

Reason for this pull request

Release process for 1.8.x series changes after 1.9.0 release:

• will release off develop-1.8 instead of develop.
• keeps sync with stable-1.8 instead of stable.

Plus changes from #1691

Merge after 1.9.0 release

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

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📚 Documentation preview 📚: https://datacube-core--1692.org.readthedocs.build/en/1692/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.78%. Comparing base (22cced7) to head (eed0df9).
Report is 1 commits behind head on develop-1.8.

Additional details and impacted files

@@             Coverage Diff              @@
##           develop-1.8    #1692   +/-   ##
============================================
  Coverage        91.78%   91.78%           
============================================
  Files              132      132           
  Lines            14646    14646           
============================================
  Hits             13443    13443           
  Misses            1203     1203           

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