Fix: Require tests on PHP 8.0 to pass

[localheinz]

This PR

• requires tests on PHP 8.0 to pass

Follows #172.

[codecov]

Codecov Report

Merging #173 into main will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff             @@
##                main      #173   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity        20        20           
===========================================
  Files              1         1           
  Lines             53        53           
===========================================
  Hits              53        53           

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