This replaces the `migrate` dashicon with `external` in the section description styles. Follow-up to [38829]. Props pross. Fixes #55542. Built from https://develop.svn.wordpress.org/trunk@53099 git-svn-id: http://core.svn.wordpress.org/trunk@52688 1a063a9b-81f0-0310-95a4-ce76da25c4cd