Replace Travis with GitHub Actions

[acorncom]

With Travis CI increasingly unmaintained (and their free tier for OSS usage gone), thought it'd be good to have tests running again in a way that's visible to the broader community.

This PR:

• sets up a basic Github action (doesn't yet have all the bells / whistles that were setup with Travis)
• cleans up errors and depreciation warnings thrown by PHPUnit
• tests against PHP 8.3
• updates the status badge

Things that either need to be done on this PR or follow-on PRs:

• setup smoke tests (not currently setup here, requires removal flags)
• allow failing PHP nightly builds
• re-enable PHP 5.5 tests without unit tests (do we care about PHP 5.5 any more?)
• setup PHP coveralls uploads (and token?)

Happy to help out with the pieces that still remain here but wanted to open this PR to see if there was actual interest in merging this before doing more work