Enhancement: Use Php73 rule set

[localheinz]

This pull request

• uses the Php73 rule set
• runs make coding-standards

[codecov]

Codecov Report

Merging #331 (7171347) into main (e821e60) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main     #331   +/-   ##
=========================================
  Coverage     93.18%   93.18%           
  Complexity       88       88           
=========================================
  Files            11       11           
  Lines           176      176           
=========================================
  Hits            164      164           
  Misses           12       12           

Impacted Files	Coverage Δ
src/Exception/CouldNotSet.php	100.00% <100.00%> (ø)
src/Exception/CouldNotUnset.php	100.00% <100.00%> (ø)
src/Exception/NotBackedUp.php	100.00% <100.00%> (ø)
src/Exception/NotSet.php	100.00% <100.00%> (ø)
src/Exception/ShouldNotBeUsed.php	100.00% <100.00%> (ø)
src/SystemVariables.php	92.85% <100.00%> (ø)
src/TestVariables.php	82.75% <100.00%> (ø)

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