Clean up the Test Double generator and runtime

[sebastianbergmann]

This closes #5246, #5247, #5248, #5249, #5250, #5310, #5311, #5312, #5313, #5314, #5316, #5321, #5416, #5424, and #5536 as well as deletes now unused code and simplifies existing code.

[codecov]

Codecov Report

Attention: Patch coverage is 95.62044% with 6 lines in your changes missing coverage. Please review.

Project coverage is 94.50%. Comparing base (0e87363) to head (753eda6).
Report is 205 commits behind head on main.

❌ We are unable to process any of the uploaded JUnit XML files. Please ensure your files are in the right format.

Files with missing lines	Patch %	Lines
src/Framework/MockObject/Generator/MockBuilder.php	90.38%	5 Missing ⚠️
src/Framework/MockObject/Generator/Generator.php	97.50%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #5697      +/-   ##
============================================
- Coverage     94.66%   94.50%   -0.16%     
+ Complexity     6733     6613     -120     
============================================
  Files           712      700      -12     
  Lines         20305    19778     -527     
============================================
- Hits          19221    18692     -529     
- Misses         1084     1086       +2     

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