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

• Links to any other associated PRs
  The root cause is fixed in this PR: Fix service instance operations duplicates root cause #2916
  Add an unique index to service_instance_operations #2929

• I have reviewed the contributing guide

• I have viewed, signed, and submitted the Contributor License Agreement

• I have made this pull request to the main branch

• I have run all the unit tests using bundle exec rake

• I have run CF Acceptance Tests

[philippthun]

The newly added test passes even without the migration. The reason for this is the following line in app/models/services/service_instance.rb:

add_association_dependencies service_instance_operation: :destroy

I would propose to remove this 'association dependency' (then the test should fail without the new migration). This would mean that service instance operations would be consistent with service binding operations, route binding operations and service key operations - they also do not have such an 'association dependency' and use DELETE CASCADE instead.