[CMSP-721] Ensure Behat tests are run on PHP 8.3 site environment

[jazzsequence]

This PR doesn't do much in itself. Most of the work of ensuring the Behat tests run on PHP 8.3 happen in the actual CI fixture site. However, there are a number of minor changes made here to ensure that the tests run correctly:

• removes unused PHP Unit test job from .circleci/config.yml
• bumps the tested-up-to to 6.4 to align with the updated fixture site
• outputs the PHP version from terminus env:info to ensure we're testing on PHP 8.3
• ignores a NewRelic error when validating the tested-up-to version (see LOPS-1909) fixed by https://github.com/pantheon-systems/infrastructure/pull/3574

[rwagner00]

I don't see anything here that looks related to New Relic, but I may be missing something. Approved with that note.