Update phpunit to have version constraint in workflow matrix

[thelovekesh]

Summary

Fixes https://github.com/WordPress/performance/actions/runs/21458842709/job/61828880918?pr=2365

Relevant technical choices

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[github-actions]

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Co-authored-by: thelovekesh <thelovekesh@git.wordpress.org>
Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.17%. Comparing base (509ad26) to head (a96cb61).
⚠️ Report is 6 commits behind head on trunk.

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk    #2373   +/-   ##
=======================================
  Coverage   69.17%   69.17%           
=======================================
  Files          90       90           
  Lines        7708     7708           
=======================================
  Hits         5332     5332           
  Misses       2376     2376           

Flag	Coverage Δ
multisite	69.17% <ø> (ø)
single	35.39% <ø> (ø)

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