Add inline-relations-depth to service_plans query in CLI.

- This is needed to make sure that the necessary data about the service plans and their associated
services is returned to the CLI.
- [#63080352]
Improve messaging for migrate-service-instances

 - Add the warning message
 - Include the number of instances that will be migrated in the prompt
 - Refactor tests
 - [#63080352]
Use /v2/services with a filter to find plans

- Remove ServiceRepository.FindServicePlanToMigrateByDescription
- Add the more generic ServiceRepository.FindServicePlanByDescription
- [#63080352]