GitHub Actions: Use Composite Actions and Shared Workflow

[n7studios]

Summary

With the growth of the GitHub actions, this PR attempts to simplify by:

• using composite actions for steps shared across different workflows,
• adds a reusable Dependabot workflow
• adds a reusable Test workflow, which can be used by both the "Test" and "Backward Compat. Test" workflows.

Testing

Existing tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)