[BUGFIX] Add maxitems for field "metadata" in TCA

Value "maxitems" => 1 must be added to field
`metadata` since it has a one-to-one relation.

Change-Id: Ie8771809b952e3b2aa441fcb222a6a3db301655e
Releases: 6.2
Resolves: #55411
Reviewed-on: https://review.typo3.org/27112
Reviewed-by: Wouter Wolters
Reviewed-by: Xavier Perseguers
Reviewed-by: Frans Saris
Reviewed-by: Stefan Froemken
Reviewed-by: Ingo Schmitt
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny