Admin Page: Fix the Release Announcement Modal's top post margin on small devices

#### Describe the solution you'd like
On small devices, the Release Announcement Modal's top margin is very small. It would be nice to make it equal to the left/right margins, as described in this [comment](https://github.com/Automattic/jetpack/pull/13710#issuecomment-610904316).

Start from the Admin Page's Release Announcement Modal and review the linked comment for the intended spacing behavior on small devices. Confirm the modal's top margin matches its left and right margins, then verify the result at small-device widths.

Written by the indexing model from the issue text.