Twenty Twenty-One: Fixes List Item Block Margin.

The margin was different depending on the front or back of the editor. This patch is a companion to the next commit incoming from issue 63549. That issue resolves the line-height inconsistency, and this one does not.
  
Props nidhidhandhukiya, ugyensupport, sandeepdahiya, rishabhwp, mleraygp, SirLouen, sabernhardt.  
Fixes #60196.


git-svn-id: https://develop.svn.wordpress.org/trunk@60366 602fd350-edb4-49c9-b593-d223f7449a82

Author: karmatosed

src/wp-content/themes/twentytwentyone/assets/css/style-editor.css

@@ -2648,3 +2648,6 @@ a {
 .has-red-to-purple-gradient-background {
 	background: linear-gradient(160deg, var(--global--color-red), var(--global--color-purple));
 }
+.wp-block-list [data-block] {
+	margin: 0;
+}