A coverage driver change that broke one CI image

PHPUnit 11 dropping support for one coverage driver, on a CI image that had it and not the other.

- uses: shivammathur/setup-php@v2
  with:
    php-version: '8.3'
    coverage: pcov        # was: xdebug

# and the reason to have been on pcov all along:
#   xdebug coverage    4m 41s
#   pcov coverage      1m 12s
# on the same suite.

The upgrade forced a change that was overdue for an unrelated reason, which is the useful kind of forced change. Xdebug remains the one to have locally for stepping, and having both in a CI image is a way of making the runner slower for no benefit — the driver is chosen per job and the jobs have different needs.