Drop PHP 8.1

[samuelwei]

• Drop PHP 8.1
• Bump dev tools
  • phpstan: v1 -> v2
  • phpunit: v11 -> v12
  • rector: v1 -> v2
  • psalm: v5 -> v6

[codecov]

Codecov Report

❌ Patch coverage is 97.22222% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.72%. Comparing base (b806569) to head (cc46c56).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Responses/BaseResponseAsJson.php	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #248      +/-   ##
============================================
- Coverage     98.09%   96.72%   -1.38%     
- Complexity      454      474      +20     
============================================
  Files            54       57       +3     
  Lines          1049     1098      +49     
============================================
+ Hits           1029     1062      +33     
- Misses           20       36      +16     

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