fix(auth-modal): handle very long newsletter lists list

[adekbadek]

All Submissions:

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

Changes proposed in this Pull Request:

Handle very long lists of newsletter lists.

See 1200133283036252-as-1205829529414334

How to test the changes in this Pull Request:

1. On master, go to Engagement -> Newsletters -> Subscription Lists and edit a list description to something super long. Can be your favourite book, for example.
2. Visit the site, click on the "My Account" in top-right corner, then "I don't have an account"
3. Observe the lists are making the modal assume the window height, with the CTAs pushed below the fold
4. Switch to this branch, observe the newsletters lists list is scrollable

Other information:

• 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?

[matticbot]

🎉 This PR is included in version 2.11.1-alpha.1 🎉

The release is available on GitHub release

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[matticbot]

🎉 This PR is included in version 2.11.2-alpha.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[matticbot]

🎉 This PR is included in version 2.11.2 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀