Migrating from custom to google managed IAP client #2924
Description
With the deprecation of IAP OAuth Admin API, we need to use google managed oauth clients instead of custom oauth client.
Google managed oauth clients can be provisioned by excluding the spec.iap.oauthclientCredentials section in the BackendConfig.
However, for already provisioned BackendConfigs, removing this doesn't configure the BackendConfig to use the google managed oauth client rather the original oauth client continues to be used.