Update URL Metric storage REST API endpoint to return status code 423 Locked instead of 403 Forbidden

[hbhalodia]

Summary

Fixes #1860

Relevant technical choices

• Updated the permission_callback for url-metrics:store route to use 423 as status code instead of 403.

[github-actions]

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Co-authored-by: hbhalodia <hbhalodia@git.wordpress.org>
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[westonruter]

Thank you!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.64%. Comparing base (cb79463) to head (647c173).
Report is 2 commits behind head on trunk.

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk    #1863   +/-   ##
=======================================
  Coverage   66.64%   66.64%           
=======================================
  Files          88       88           
  Lines        7015     7015           
=======================================
  Hits         4675     4675           
  Misses       2340     2340           

Flag	Coverage Δ
multisite	66.64% <100.00%> (ø)
single	37.27% <0.00%> (ø)

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