ci: add PHP 8.6

[phil-davis]

leaving the other test matrices for PHP unchanged for now.

Run all CI checks on PHP 8.6.

This is just a draft PR to check that CI passes. When PHP 8.6 gets to the RC stage, then I will make a real PR to add 8.6.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.82%. Comparing base (59b0476) to head (d5a8a51).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #287      +/-   ##
============================================
+ Coverage     93.98%   94.82%   +0.84%     
+ Complexity      263      260       -3     
============================================
  Files            15       15              
  Lines           864      851      -13     
============================================
- Hits            812      807       -5     
+ Misses           52       44       -8     

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