[FEATURE] Increase size of be_groups table subgroup field

Change field subgroup from varchar(255) to text to allow
storing more subgroups.

Resolves: #36505
Releases: 6.2
Change-Id: Iec8cbc3fa83d03a891d4854fcf67c2418ef39d27
Reviewed-on: https://review.typo3.org/21593
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe