Administration: Use rounded corners for dashboard widgets drop zones.

Admin Cards and Metabox were modified in [61646] to better align with the modern WordPress design system, but dashboard widgets drop zones weren't updated as such. This changeset adds rounded corners to these drop zones for better consistency.

Follow-up to [61646].

Props ocean90, niravsherasiya7707, stefanvelthuys, valentingrenier, manhar, huzaifaalmesbah, r1k0.
Fixes #64787.
See #64549.



git-svn-id: https://develop.svn.wordpress.org/trunk@61877 602fd350-edb4-49c9-b593-d223f7449a82