[BUGFIX] Form email validation is case sensitive

Currently the FORM email validation is using a regular expression which
checks the email address. This check also inclused case sensitivity.

This patch fixes this by using the email address validator which comes
with the extension core.

Change-Id: Ib1449198f66f023a01c9791e74eb2d634abd77df
Resolves: #43924
Releases: 6.0, 6.1, 6.2
Reviewed-on: https://review.typo3.org/20525
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters