Provide default for CPU boost annotation

From https://gitlab.developers.cam.ac.uk/uis/devops/regent-house-ballots/infrastructure/-/jobs/1163963#L149, Google implicitly add the run.googleapis.com/startup-cpu-boost annotation to the metadata which causes terraform plan to always complain that the plan is out of date.

Add a default for run.googleapis.com/startup-cpu-boost since, otherwise, the terraform plan always thinks changes need to be made:

Terraform will perform the following actions:
  # module.webapp.google_cloud_run_service.webapp will be updated in-place
  ~ resource "google_cloud_run_service" "webapp" {
        id                         = "locations/europe-west2/namespaces/rh-ballots-devel-d15e5382/services/webapp"
        name                       = "webapp"
        # (4 unchanged attributes hidden)
      ~ template {
          ~ metadata {
              ~ annotations = {
                  - "run.googleapis.com/startup-cpu-boost"  = "true" -> null
                    # (6 unchanged elements hidden)
                }
                # (2 unchanged attributes hidden)
            }
            # (1 unchanged block hidden)
        }
        # (2 unchanged blocks hidden)
    }
Plan: 0 to add, 1 to change, 0 to destroy.