Bump WordPress "tested up to" version 6.6

[jeffpaul]

Description of the Change

This PR bumps the WP tested-up-to 6.6, the minimum to 6.4, and updates the e2e tests to using 6.4 as the minimum as well. It also removes the table of contents from the readme now that GitHub auto-generates that.

Closes #316.

How to test the Change

Use Plugin ZIP action to get version from this PR and run through critical flows, otherwise review results of e2e tests.

Changelog Entry

Changed - Bump WordPress "tested up to" version 6.6. Changed - Bump WordPress minimum supported version to 6.4.

Credits

Props @sudip-md, @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.