NET-6664 Specify cluster scope for MeshGateway CRD

[nathancoleman]

Changes proposed in this PR:

• Change the scope of the MeshGateway CRD to Cluster due to the fact that the resource is partition-scoped in Consul and thus should not have a namespace

How I've tested this PR:
Using this setup

$ helm upgrade --install consul <path_to_local_main_checkout>/charts/consul --values ./values.yaml
$ kubectl apply -f gateway.yaml

Instead of the namespace declaration error previously caused by the Namespaced scope, you'll see an error in the controller logs indicating that the create/update function is not yet implemented, which is expected.

ERROR    Reconciler error    {"controller": "meshgateway", "controllerGroup": "mesh.consul.hashicorp.com", "controllerKind": "MeshGateway", "MeshGateway": {"name":"mesh-gateway"},  │
│ "namespace": "", "name": "mesh-gateway", "reconcileID": "c3917aab-878d-48c5-adbc-d701bab04c1b", "error": "onCreateUpdate not implemented"}

How I expect reviewers to test this PR:
See above

Checklist:

• Tests added
• CHANGELOG entry added