[TASK] Use &route=/ajax/ instead of &ajaxId for BE AJAX calls

In order to streamline backend usage even further, the AJAX Request Handler
is now using the "route" parameter the same way.

This way, the RouteDispatcher does not have to distinguish between the BE
calls anymore, and the Form Protections are streamlined.

Resolves: #81899
Releases: master
Change-Id: I48bf2406eaff2316d3f0fe5dc631a51067a570f6
Reviewed-on: https://review.typo3.org/53459


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Tested-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>