ci: drop PHP 8.1 and add PHP 8.5

[owenvoke]

Changes proposed in this pull request:

• Drop PHP 8.1
• Add PHP 8.5 testing
• Update GitHub Actions dependencies

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.14%. Comparing base (a67f194) to head (9685f8b).
⚠️ Report is 10 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #240      +/-   ##
============================================
+ Coverage     55.10%   55.14%   +0.04%     
- Complexity       78       82       +4     
============================================
  Files            10       10              
  Lines           245      243       -2     
============================================
- Hits            135      134       -1     
+ Misses          110      109       -1     

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