Dropped support for PHP 7.4, and added experimental builds on PHP 8.1

[acelaya]

Closes #118
Closes #117
Closes #120

[codecov-commenter]

Codecov Report

Merging #119 (041b1de) into develop (fa6a4ca) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##             develop     #119      +/-   ##
=============================================
- Coverage      98.03%   97.97%   -0.06%     
  Complexity       195      195              
=============================================
  Files             63       63              
  Lines            510      495      -15     
=============================================
- Hits             500      485      -15     
  Misses            10       10              

Impacted Files	Coverage Δ
src/Command/AbstractInstallCommand.php	96.29% <ø> (-0.48%)	⬇️
src/Command/SetOptionCommand.php	100.00% <ø> (ø)
src/Config/ConfigGenerator.php	96.87% <ø> (-0.35%)	⬇️
src/Factory/ApplicationFactory.php	90.00% <100.00%> (+2.50%)	⬆️
src/Model/ImportedConfig.php	100.00% <100.00%> (ø)
src/Service/InstallationCommandsRunner.php	100.00% <100.00%> (ø)
src/Service/ShlinkAssetsHandler.php	100.00% <100.00%> (ø)
src/Config/Util/ConfigOptionsValidatorsTrait.php	100.00% <0.00%> (ø)

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