Raise PHP version to ^8.0 + Add rector

[xepozz]

Added rector/rector to CI and composer.json. Mass creation refers to yiisoft/yii-dev-tool#232

[codecov]

Codecov Report

Base: 79.13% // Head: 79.36% // Increases project coverage by +0.23% 🎉

Coverage data is based on head (e463bfd) compared to base (1a71a8a).
Patch coverage: 86.66% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #64      +/-   ##
============================================
+ Coverage     79.13%   79.36%   +0.23%     
+ Complexity      161      160       -1     
============================================
  Files            12       12              
  Lines           484      475       -9     
============================================
- Hits            383      377       -6     
+ Misses          101       98       -3     

Impacted Files	Coverage Δ
src/ErrorHandler.php	60.00% <ø> (+0.29%)	⬆️
src/Event/ApplicationError.php	0.00% <0.00%> (ø)
src/Exception/ErrorException.php	46.15% <0.00%> (ø)
src/ErrorData.php	100.00% <100.00%> (ø)
src/Middleware/ErrorCatcher.php	100.00% <100.00%> (ø)
src/Middleware/ExceptionResponder.php	100.00% <100.00%> (ø)
src/Renderer/HeaderRenderer.php	100.00% <100.00%> (ø)
src/Renderer/HtmlRenderer.php	80.00% <100.00%> (ø)
src/Renderer/JsonRenderer.php	100.00% <100.00%> (ø)
src/Renderer/PlainTextRenderer.php	100.00% <100.00%> (ø)
... and 7 more

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