Raise PHP version to 8.1

[Tigrov]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌
Fixed issues	-

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.69%. Comparing base (06226eb) to head (d3ed7a2).
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #347      +/-   ##
============================================
- Coverage     98.71%   98.69%   -0.02%     
  Complexity      174      174              
============================================
  Files            15       15              
  Lines           546      538       -8     
============================================
- Hits            539      531       -8     
  Misses            7        7              

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