For themes without non-block themes (i.e. without a `/block-templates/index.html` file), restores the "Customize" menu item under "Appearance" menu to its original location of `6`.
For block themes, moves it to position `8`, as "Styles" is in position `7` as of [52158].
Follow-up to [29026], [52069], [52158].
Props poena, davidbaumwald, sabernhardt, hellofromTonya.
Fixes#54418.
Built from https://develop.svn.wordpress.org/trunk@52178
git-svn-id: http://core.svn.wordpress.org/trunk@51770 1a063a9b-81f0-0310-95a4-ce76da25c4cd