[TASK] Deprecate methods related to traditional AJAX request dispatching

The execution of traditional AJAX requests not registered
via routes is already deprecated with #73352, however
the registration and usage of the methods to add AJAX
endpoints haven't been deprecated yet.

The patch takes care of deprecating
    - BackendUtility->getAjaxUrl()
    - UriBuilder->buildUriFromAjaxId()

Resolves: #75340
Releases: master
Change-Id: I6adf7f7c868c87f08400aa548d5deb8febddd857
Reviewed-on: https://review.typo3.org/47466


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>