Prepare for 1.9.21 release

[SpacemanPaul]

Reason for this pull request

Bug fix to STAC handling required for Explorer, and starting to raise minimum Python requirement for ODC packages to 3.12.

Proposed changes

• Bump fallback version in pyproject.toml

• Update uv.lock

• Add 1.9.21 release notes.

• Tests added / passed

• Pull Request Title will make sense in ODC Release Notes

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📚 Documentation preview 📚: https://opendatacube--2515.org.readthedocs.build/en/2515/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.51%. Comparing base (2f0bf52) to head (361f587).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2515   +/-   ##
========================================
  Coverage    86.51%   86.51%           
========================================
  Files          162      162           
  Lines        17277    17277           
========================================
  Hits         14948    14948           
  Misses        2329     2329           

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