[TASK] Store System Maintenance tokens in data attributes

The tokens used in the System Maintenance area are stored in hidden
`<div>` elements, which is syntactically wrong and considered bad
practice.
Instead, the tokens are now stored in data attributes.

Resolves: #85455
Releases: master
Change-Id: I3a3be24897caa56c20355f0d36c7e7c4ae702640
Reviewed-on: https://review.typo3.org/57451


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: Mathias Brodala <mbrodala@pagemachine.de>