[TASK] Extend DB fields for be users/groups of DB/file mounts

The maxitems entry for be_users / be_groups fields
db/file mounts was raised in issue #55018. The DB schema of
these fields should reflect the changes as well.

Releases: 6.2
Followup to: #55018
Resolves: #55025

Change-Id: I475ed18d3210a68e476c1434fa83b0b410937c8a
Reviewed-on: https://review.typo3.org/26837
Reviewed-by: Wouter Wolters
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel