[BUGFIX] Use validEmail() instead of deprecated checkEmail()

Change-Id: I72aa69adc75820cf513bb87cf2af6ea1b50a2fc7
Resolves: #57934
Releases: 4.5
Reviewed-on: https://review.typo3.org/29489
Reviewed-by: Oliver Klee
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind