[BUGFIX] Unified maxitems default value for group fields

The maxitems value for a group field defaults to 1 instead of 100000 so
that it is consistent with the TCA reference and the validation in the
DataHandler.

Resolves: #55731
Releases: 6.2
Change-Id: I517e45a9227ab9d971a9e7bc4410498319192517
Reviewed-on: https://review.typo3.org/27378
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters