Add PHP 8.1 to CI

[ker0x]

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[codecov]

Codecov Report

Merging #18 (3079506) into main (14cc8ea) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main      #18   +/-   ##
=========================================
  Coverage     97.44%   97.44%           
  Complexity      193      193           
=========================================
  Files            26       26           
  Lines           586      586           
=========================================
  Hits            571      571           
  Misses           15       15           

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