Update Support Level from `Beta` to `Stable`

[jeffpaul]

Description of the Change

This PR changes the support level for Block Catalog from Beta to Stable to catch up with the current state of the plugin.

How to test the Change

Use a markdown previewer to see readme changes.

Changelog Entry

Added - New feature Changed - Update Support Level from Beta to Stable.

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.