Bump WordPress "tested up to" version 6.5

[jeffpaul]

Description of the Change

This PR bumps the WP tested-up-to version 6.5, bumps the WP min to 6.3, and removes unnecessary headers from readme.txt. This also removes the ISSUE_TEMPLATES as those should be pulled in from the org-wide .github repo.

Closes #280.

How to test the Change

Changelog Entry

Changed - Bump WordPress "tested up to" version 6.5. Changed - Bump WordPress minimum from 5.7 to 6.3.

Credits

Props @qasumitbagthariya, @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.