Manage all GCP project metadata in Terraform

Context

As a corrective action from production#5489 (closed), we noticed that we currently did not manage GCP project metadata fully through Terraform.

We do have some individual resources managing specific metadata keys, but that doesn't generate a diff when other keys are added or removed.

Proposal

Replace use of compute_project_metadata_item with compute_project_metadata in https://ops.gitlab.net/gitlab-com/gl-infra/terraform-modules/google/project