Cancel checks for superseded PR commits

[anth-volk]

Fixes #3853

Summary

• group API v1 pull request workflow runs by workflow and pull request number
• cancel the in-progress validation run when the same pull request receives another commit
• leave deployment workflow concurrency unchanged
• add the required changelog fragment

Verification

• parsed .github/workflows/pr.yml successfully as YAML
• git diff --check passed
• confirmed that pushing commit 8346ff02 cancelled the queued workflow for superseded commit 1c1b5079

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.48%. Comparing base (61883c4) to head (4357518).

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3854       +/-   ##
===========================================
+ Coverage   46.37%   87.48%   +41.10%     
===========================================
  Files         178      193       +15     
  Lines       10421    11538     +1117     
  Branches     1759     2008      +249     
===========================================
+ Hits         4833    10094     +5261     
+ Misses       5199      882     -4317     
- Partials      389      562      +173     

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