Update MetalLB version (#6139)

If running MetalLB v0.7.3 on k8s v1.18.2, metallb pods output the
following parsing error of v1.ServiceList:

  $ kubectl logs controller-dbb46cf84-fw8h8 -n metallb-system
  {
    "caller":"reflector.go:205",
    "level":"error",
    "msg":"go.universe.tf/metallb/internal/k8s/k8s.go:231:
      Failed to list *v1.Service: v1.ServiceList:
        Items: []v1.Service: v1.Service: ObjectMeta:
        v1.ObjectMeta: readObjectFieldAsBytes:
        expect : after object field, parsing 1605

Then an external IP address is never allocated to the Service of
LoadBalancer type.
By updating MetalLB version to the latest v0.9[1] today, this issue
can be solved.

[1]: https://hub.docker.com/r/metallb/controller/tags

contrib/metallb/roles/provision/defaults/main.yml

@@ -13,4 +13,4 @@ metallb:
     cpu: "100m"
     memory: "100Mi"
   port: "7472"
-  version: v0.7.3
+  version: v0.9.3