dev: Bump the PHPStan testing to PHP 8.4

[johnbillion]

PHP unit tests are run on PHP 8.4 so PHPStan should check in the context of 8.4 too.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.84%. Comparing base (c9abf12) to head (51cb4c0).
⚠️ Report is 1 commits behind head on trunk.

Additional details and impacted files

@@            Coverage Diff            @@
##              trunk      #82   +/-   ##
=========================================
  Coverage     83.84%   83.84%           
  Complexity       96       96           
=========================================
  Files             8        8           
  Lines           520      520           
=========================================
  Hits            436      436           
  Misses           84       84           

Flag	Coverage Δ
unit	83.84% <ø> (ø)

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[github-actions]

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Co-authored-by: johnbillion <johnbillion@git.wordpress.org>
Co-authored-by: JasonTheAdams <jason_the_adams@git.wordpress.org>

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[JasonTheAdams]

Good catch!