Fix margin on tiled galleries

[gravityrail]

Fixes TBD

Changes proposed in this Pull Request:

Is this a new feature or does it add/remove features to an existing part of Jetpack?

• If you're an Automattician, include a shortlink to the p2 discussion with Jetpack Product here.

Testing instructions:

• Go to '..'

Proposed changelog entry for your changes:

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello gravityrail! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer, review, and approve D36825-code before merging this PR. Thank you!

[jetpackbot]

	Warnings
⚠️	The PR is missing at least one [Status] label. Suggestions: [Status] In Progress, [Status] Needs Review

This is an automated check which relies on PULL_REQUEST_TEMPLATE. We encourage you to follow that template as it helps Jetpack maintainers do their job. If you think 'Testing instructions' or 'Proposed changelog entry' are not needed for your PR - please explain why you think so. Thanks for cooperation 🤖

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[kraftbj]

Before you dive too deep, where are you seeing the margins incorrect? There is this PR staged for merge wrt TGs and Twenty Twenty: #14224

[gravityrail]

Dupe of #14224