Bump PHP version to 8.1

[vjik]

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.66%. Comparing base (95983b9) to head (2f0538b).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #90   +/-   ##
=========================================
  Coverage     91.66%   91.66%           
  Complexity       69       69           
=========================================
  Files             1        1           
  Lines           168      168           
=========================================
  Hits            154      154           
  Misses           14       14           

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