Document the steps required to migrate to the latest version of the gcp-deploy-boilerplate

Following the release of v9.0 of our Cloud Run App Terraform module the gcp-deploy-boilerplate has been updated to make use of the new features. This issue is to document - in the form of a how-to - the steps required to migrate both trivial and non-trivial cloud run deployments from previous versions of the boilerplate to the latest. This will ideally include steps for a situation where down time is acceptable and steps for when it is not.