Fix: Drop support for PHP 7.3

[localheinz]

This pull request

• drops support for PHP 7.3

Fixes #838.

[codecov]

Codecov Report

Merging #852 (d4f458b) into main (c172e90) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main     #852   +/-   ##
=========================================
  Coverage     87.81%   87.81%           
  Complexity       57       57           
=========================================
  Files             4        4           
  Lines           279      279           
=========================================
  Hits            245      245           
  Misses           34       34           

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