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 98.80%. Comparing base (d21a9ac) to head (b6e0b2c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #356      +/-   ##
============================================
- Coverage     98.80%   98.80%   -0.01%     
  Complexity      288      288              
============================================
  Files            25       25              
  Lines           840      839       -1     
============================================
- Hits            830      829       -1     
  Misses           10       10              

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