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 99.17%. Comparing base (03ea383) to head (62f65d3).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #164   +/-   ##
=========================================
  Coverage     99.17%   99.17%           
  Complexity      111      111           
=========================================
  Files             7        7           
  Lines           364      364           
=========================================
  Hits            361      361           
  Misses            3        3           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.