fix: Do not ignore "mesh_id" label on "google_container_cluster" resource

[legal90]

Fixes #1671
Relates to #1061

According to the documentation, mesh_id label is supposed to be set explicitly on the GKE cluster:

• [asm module doc]

  terraform-google-kubernetes-engine/modules/asm/README.md

  Lines 36 to 43 in d82c50c

  	Note that the [`mesh_id` label on the cluster](https://cloud.google.com/service-mesh/docs/managed/auto-control-plane-with-fleet#apply_the_mesh_id_label) is required for metrics to get displayed on the Anthos Service Mesh pages in the Cloud console (Topology, etc.). Illustrated with the full example mentioned above, here is an example of what your cluster should have:
  	```tf
  	module "gke" {
  	...
  	cluster_resource_labels = { "mesh_id" : "proj-${data.google_project.project.number}" }
  	...
  	}

• [Google doc for managed ASM] https://cloud.google.com/service-mesh/docs/managed/provision-managed-anthos-service-mesh#configure_your_fleet
• [Google doc for in-cluster ASM] https://cloud.google.com/service-mesh/docs/unified-install/project-cluster-setup#set_up_your_cluster

However, right now this label is ignored by terraform, so it's not possible to set it using IaC.
This PR fixes that issue and removes mesh_id label from the ignore_changes list

[apeabody]

/gcbrun

[apeabody]

/gcbrun

[apeabody]

/gcbrun

[apeabody]

/gcbrun

[apeabody]

/gcbrun

[apeabody]

Thanks for the contribution @legal90!

Thanks for the review @ferrarimarco!