[TASK] Deprecate `T3_RETURN_URL`

The global JavaScript variable `T3_RETURN_URL` containing the returnUrl
sent either via GET or POST has been marked as deprecated.

Resolves: #88862
Releases: master
Change-Id: I77e94932ef117428100dcd413380940ff33a9b9a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/61390


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>