Bump PHP to 8.1-8.4 + Enable BC checker

[vjik]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.40%. Comparing base (e0c83d1) to head (bbeb8a3).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #44      +/-   ##
============================================
+ Coverage     91.95%   95.40%   +3.44%     
  Complexity       34       34              
============================================
  Files             3        3              
  Lines            87       87              
============================================
+ Hits             80       83       +3     
+ Misses            7        4       -3     

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To fix the problem, you should add a permissions block that explicitly limits the permissions available to the workflow. The best approach is to add this block at the workflow (top) level unless you know the job requires more elevated permissions than read-only. Since the job appears to merely check backward compatibility via a reusable workflow and likely does not need to write to the repository or create/update pull requests, a minimal contents: read permission is the least privileged and safest starting point. You should insert the following block near the top of the file, immediately after the name: property (line 25), so it applies to all jobs in the workflow:

permissions:
  contents: read

No additional methods, definitions, or library installations are necessary—just this single insertion.