Course: Change col target_group to type text in table crs_settings

[lscharmer]

This PR fixes a problem in the event table when migrating from utf8mb3 to utf8mb4.

Currently the table cannot be converted to utf8mb4 because the row size would be to large:

Error: query: Row size too large. The maximum row size for the used table type, not counting BLOBs, is 65535. This includes storage overhead, check the manual. You have to change some columns to TEXT or BLOBs
  SQLSTATE: 42000

With utf8mb4 4 bytes per char instead 3 are reserved, so each (var-)char field length is multiplied by 4 instead 3.

It is enough to change one varchar(4000) field to type text to fit into the row size limit.

This PR is in preparation for an upcoming PR to change the Database charset & collation from utf8mb3 to utf8mb4.