Administration: Improve dashboard widgets border styles.

This changeset fixes a CSS glitch on dashboard widgets bottom border when they are collapsed.

Follow-up to [61646].

Props pratik-jain, audrasjb, ankitkumarshah.
Fixes #65017.
See #64549.



git-svn-id: https://develop.svn.wordpress.org/trunk@62206 602fd350-edb4-49c9-b593-d223f7449a82