Bump WordPress "tested up to" version 6.3 on `trunk`

[jeffpaul]

Description of the Change

This PR merges the WP "tested up to" version bump from #140 into the trunk branch to hopefully trigger the Readme Updater action and update directly on WP.org without needing a minor plugin release here on GitHub.

Closes #131.

How to test the Change

Changelog Entry

n/a

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.