fix(reader-registration-block): fix styles of newsletter lists

[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:

Fixes a styling issue.

How to test the changes in this Pull Request:

1. On trunk, ensure you have about five lists configured in the Enagement -> Newsletters settings, add descriptions to each to inflate the character count
2. Add a Reader Registration block somewhere, enable "Enable newsletter subscription" in the "Newsletter Subscription" sidebar panel, toggle all available lists, publish
3. Visit the front-end, observe the lists are cut off in the UI, not allowing to scroll up fully
4. Switch to this branch, observe issue is gone

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?

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[leogermani]

I could not confirm the issue on my local (I had way more than 5 lists).. but after this things still look good

[github-actions]

Hey @, good job getting this PR merged! 🎉

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