Cleanup release notes

[tpvasconcelos]

...and add a ./cicd_utils/find-unmentioned-prs.sh helper script to find merged PRs that were not mentioned in the changelog yet

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📚 Documentation preview 📚: https://ridgeplot--334.org.readthedocs.build/en/334/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (476c599) to head (afd98b1).
⚠️ Report is 110 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #334   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           18        18           
  Lines          655       655           
  Branches        83        83           
=========================================
  Hits           655       655           

Flag	Coverage Δ
combined-src	99.84% <ø> (ø)

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