Update above-header hook used by plugin

[laurelfulford]

All Submissions:

• [x] Have you followed the Newspack Contributing guideline?
• [x] Does your code follow the WordPress' coding standards and VIP Go coding standards?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

This PR updates the before_header used by the plugin for the more standard wp_body_open, which is already included in our classic theme and that appears by default in block themes.

This fixes a couple small issues with this plugin + block themes, and hopefully doesn't cause any weirdness with our existing theme.

I think this is a pretty harmless change -- the two hooks are stacked in our current theme -- but definitely would appreciate an eye to potential issues!

Closes 1203518336360239-as-1203518336360246, 1203518336360239-as-1203518336360244

How to test the changes in this Pull Request:

1. Using a block theme (like ours), and starting on trunk, set up an above header prompt.
2. View on the front end and note it doesn't work.
3. Set up a popup prompt.
4. View on the front-end and note that while it works, you can't close it.
5. Apply this PR.
6. Repeat steps 1-4 and confirm that both are now working.
7. Switch to our classic theme, and repeat steps 1-4 and confirm they're still okay.

Other information:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [ ] Have you written new tests for your changes, as applicable?
• [ ] Have you successfully ran tests with your changes locally?

[laurelfulford]

Thanks @dkoo! 🙌

[matticbot]

:tada: This PR is included in version 2.32.0-alpha.1 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket:

[matticbot]

:tada: This PR is included in version 2.32.0 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: