[6.0] chore: update to phpstan major version 2

[phil-davis]

Backport #255 to 6.0

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.28%. Comparing base (7d2c2a8) to head (c76b861).
⚠️ Report is 1 commits behind head on 6.0.

Additional details and impacted files

@@            Coverage Diff            @@
##                6.0     #276   +/-   ##
=========================================
  Coverage     94.28%   94.28%           
+ Complexity      262      261    -1     
=========================================
  Files            15       15           
  Lines           875      875           
=========================================
  Hits            825      825           
  Misses           50       50           

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[phil-davis]

This is difficult, there are a few changes that happened in master for 7.0 like #202 and #203 that do not so easily backport to 6.0. And actually I think that a combination of things like that were the reason for keeping major version 6, and starting a new major version 7. So I won't try to backport all this code-style and code-analysis stuff.