Add schema directive and requeue suggestion to BBM template

What does this MR do and why?

Migrations like BBM's require a gitlab schema to be set as they cannot be executed against all schema's. This MR updates the template with a recommendation to set the schema restriction. It fails out of the box, forcing any user of the template to set the schema, but at least they'll know why instead of questioning the DML statement error they get instead if they fail to add this line.

Additionally, when requeueing a BBM, it is necessary to delete the old record from the table for the new enqueue to work correctly. This detail has been forgotten a few times, so mentioning it in the template will likely reduce the frequency of this mistake and improve MR turn around time in those situations.