[FEATURE] Allow to disable "Core Updater": not only optically

Follow-up to "Allow to disable the "Core Updater" through an env-variable".
Now it's not even possible to call the individual Ajax requests that do the
update if it is disabled.

Resolves: #52639
Releases: 6.2
Change-Id: Icfa18d619bbb8aabf1896920f28fe5a63bd5c345
Reviewed-on: https://review.typo3.org/24661
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn