Enhancement: Run tests on PHP 8.1

[localheinz]

This pull request

• runs tests on PHP 8.1

[codecov]

Codecov Report

Merging #410 (c032a16) into main (2c28a46) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main     #410   +/-   ##
=========================================
  Coverage     98.88%   98.88%           
  Complexity       58       58           
=========================================
  Files            14       14           
  Lines           179      179           
=========================================
  Hits            177      177           
  Misses            2        2           

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