Twenty Fifteen: Resolves seperator block being too thick in iframe editor.

The seperator block was looking different in the iframe editor. This resolves that by unifying.

Props nidhidhandhukiya, sabernhardt, darshitrajyaguru97, yuvrajsinj2211.
Fixes #60079.

Built from https://develop.svn.wordpress.org/trunk@58231


git-svn-id: http://core.svn.wordpress.org/trunk@57694 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-content/themes/twentyfifteen/css/editor-blocks.css

@@ -806,6 +806,7 @@ p.has-drop-cap:not(:focus)::first-letter {
 /* Separator */
 
 .wp-block-separator {
+	border: 0;
 	max-width: 100px;
 }
 

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.6-alpha-58230';
+$wp_version = '6.6-alpha-58231';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.