Bump PHP version to 8.1 and PHPUnit version to 10.5.2

[arogachev]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	✔️

[what-the-diff]

PR Summary

• Upgrade to PHP 8.1
  The project’s PHP version has been updated to 8.1 to take advantage of the latest language features and performance improvements.
• Update to PHPUnit 10.5.2
  Our unit testing tool, PHPUnit, has been upgraded to version 10.5.2. This exposes us to newer testing capabilities.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (865022d) 100.00% compared to head (aba4d2d) 100.00%.
Report is 1 commits behind head on master.

❗ Current head aba4d2d differs from pull request most recent head a050cc1. Consider uploading reports for the commit a050cc1 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##              master      #217   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity       211       211           
===========================================
  Files             13        13           
  Lines            525       525           
===========================================
  Hits             525       525           

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