Enhancement: Run tests on PHP 8.1

[localheinz]

This pull request

• runs tests on PHP 8.1

[codecov]

Codecov Report

Merging #95 (b7d91bc) into main (ea9449a) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main      #95   +/-   ##
=========================================
  Coverage     99.11%   99.11%           
  Complexity       59       59           
=========================================
  Files            14       14           
  Lines           225      225           
=========================================
  Hits            223      223           
  Misses            2        2           

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