Make `service_map` variable in DA as required

1. The `service_map` variable is showing in optional variables. Add required = true in ibm_catalog.json [here](https://github.com/terraform-ibm-modules/terraform-ibm-s2s-auth/blob/0daf93bab9f3d061a7fd5643fc7a3dac802d83bd/ibm_catalog.json#L53)
2. Also, add reference to migration notes in Readme which is missing. Path to migration notes: `docs/migration_notes.md`