[SQONE-698]: Basic PHP80 support

[defunctl]

What does this do/fix?

This is an intermediate update to show how existing PHP7.4 projects can jump to PHP8.0 when their hosts force an upgrade. Essentially, this is the minimum amount of work to make this support PHP8.0 while being backwards compatible with our PHP7.4 system. A future PR will bring in PHP8.0 minimums + optimizations once Tribe Libs is also ready for that.

• Changed: Added basic support for PHP8.0.
• Removed: The Tribe Admin Dashboard plugin (not PHP8.0 compatible).
• Bumped: Tribe Libs to the 4.x major version + plugin updates.
• Removed: Any dependencies that Tribe Libs already provides in composer.json.
• Fixed: PHPCS throwing trim() errors on certain files due to bad WordPress rule typing.
• Changed: Replaced Function Mocker with Brain Monkey for Unit testing as Function Mocker isn't fully PHP8.0 compatible.
• Added: New Unit Test Class that configures Brain Monkey.
• Updated: Existing Unit tests to use Brain Monkey where they were using Function Mocker.

QA

Everything should function as before, assuming you restart the project to run on the PHP8.0 docker container.