Add delete on cascade to fk_svc_inst_op_svc_instance_id

[kathap]

We can see double entries for the same service_instance_id in the service_instance_operations table. This leads to: PG::ForeignKeyViolation: ERROR: update or delete on table "service_instances" violates foreign key constraint "fk_svc_inst_op_svc_instance_id" on table "service_instance_operations"

For service bindings, service keys and route bindings the foreign key constraint from operation to resource contains ON DELETE CASCADE; for service instances this is missing.

Adding ON DELETE CASCADE to FK fk_svc_inst_op_svc_instance_id in service_instance_operations

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