Cloudrun: Check knative observed generation when polling

[modular-magician]

Fixes: hashicorp/terraform-provider-google#8172

See description of the observedGeneration field on the REST API: https://cloud.google.com/run/docs/reference/rest/v1/namespaces.domainmappings#DomainMappingStatus

Clients polling for completed reconciliation should poll until observedGeneration = metadata.generation and the Ready condition's status is True or False.

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Release Note Template for Downstream PRs (will be copied)

cloudrun: fixed a bug where resources would return successfully due to responses based on a previous version of the resource

Derived from GoogleCloudPlatform/magic-modules#4793