Upgrade/Install: Consistent layout and accurate messages on the update screen.

* Clarifies that if you are on maintenance/security auto-updates that you are only on those and therefore there are more options available.
* Adds a message if a version control system has been detected, as automatic updates are disabled in that case.
* Ensures only one heading between `update available`, `you are on a dev version`, and `you are on latest` appears at any given time, falling back to `you are on latest` if something strange happens with the returned update data.
* Removes some older strings related to auto-updates, which greatly simplifies the above.
* Strips the `core-major-auto-updates-saved` query arg from the URL, as it is related to a dismissible notice.

Props audrasjb, pbiron, helen.
Fixes #51742.


git-svn-id: https://develop.svn.wordpress.org/trunk@49638 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -1186,6 +1186,7 @@ function wp_removable_query_args() {
 		'activated',
 		'admin_email_remind_later',
 		'approved',
+		'core-major-auto-updates-saved',
 		'deactivate',
 		'delete_count',
 		'deleted',