Fix: Drop support for PHP 7.3

[localheinz]

This pull request

• drops support for PHP 7.3

[codecov]

Codecov Report

Merging #21 (1517824) into main (a191ced) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main      #21   +/-   ##
=========================================
  Coverage     96.77%   96.77%           
  Complexity       45       45           
=========================================
  Files             9        9           
  Lines           186      186           
=========================================
  Hits            180      180           
  Misses            6        6           

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