[BUGFIX] Allow {__currentTimestamp} as form property

Resolves: #100580
Releases: main, 11.5
Change-Id: I10e4b21908a1b4649d7ab4b02c467af381017b2b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78627


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>