Enhancement: Run tests on PHP 8.1

[localheinz]

This pull request

• runs tests on PHP 8.1

[codecov]

Codecov Report

Merging #683 (e135b48) into main (1fc1810) will not change coverage.
The diff coverage is n/a.

❗ Current head e135b48 differs from pull request most recent head 3f4cf67. Consider uploading reports for the commit 3f4cf67 to get more accurate results

@@            Coverage Diff            @@
##               main     #683   +/-   ##
=========================================
  Coverage     98.48%   98.48%           
  Complexity      110      110           
=========================================
  Files            25       25           
  Lines           330      330           
=========================================
  Hits            325      325           
  Misses            5        5           

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