Update for Compatibility with PHP8.1

[bbrothers]

Description

This PR extracts the PHPUnit assertion logic into the Muzzle\Assertions\Assert class so that assertion logic can be kept to the Assert class. It also restricts the PHPUnit version to 8.0 for now while we wait for the 8.1 bug (sebastianbergmann/phpunit#3599) to be resolved.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My pull request addresses exactly one patch/feature.
• I have created a branch for this patch/feature.
• Each individual commit in the pull request is meaningful.
• I have added tests to cover my changes.