Deprecate the phpdbg option for code coverage

[stronk7]

This includes both the current default (when neither pcov or xdebug are available) and the explicit use of --coverage-phpdbg

Since PHP 8.0, phpdbg is causing all sort of problems and is not recommended. So it's time to deprecate it in order to proceed to remove it completely for moodle-plugin-ci 5.0.0.

Removal ref.: https://github.com/moodlehq/moodle-plugin-ci/issues/289

Fixes #213

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.22%. Comparing base (f1624f3) to head (cdfb095).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #298 +/- ## ========================================= Coverage 88.22% 88.22% - Complexity 732 734 +2 ========================================= Files 76 76 Lines 2267 2267 ========================================= Hits 2000 2000 Misses 267 267 ```

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[stronk7]

Just for testing purposes, this is how the deprecation warning notification is displayed whenever phpdbg is effectively used (note that only old php < 8.0 jobs can be using it, as commented, any 8.x job will fail badly).

https://github.com/stronk7/moodle-plugin-ci/actions/runs/8582321848

[stronk7]

Ta, K!