Update/wp version checker

[jeffpaul]

Description of the Change

This PR bumps the skaut/wordpress-version-checker dependency to 1.4.0 so that we can now get alerts when WordPress core hits Release Candidate for a major release if this plugin is not already set to be "tested up to" that new major version. This also adds an exception in the .gitattributes file so that we don't distribute the new .wordpress-version-checker.json file.

How to test the Change

🤷🏼 YOLO merge?

Changelog Entry

Changed - Updated the skaut/wordpress-version-checker to check WordPress "tested up to" during the Release Candidate phase.

Credits

Props @jeffpaul.

Checklist:

• [x] I agree to follow this project's Code of Conduct.
• [x] I have updated the documentation accordingly.
• [ ] I have added tests to cover my change.
• [ ] All new and existing tests pass.