Linkerd-viz: Update grafana setting when using 2.12.x

[Henry Vu]

https://github.com/linkerd/website/blob/main/linkerd.io/content/2.12/tasks/grafana.md?plain=1#L53-L74
The setting for grafana should be

grafana.ini:
  server:
    root_url: '%(protocol)s://%(domain)s:%(http_port)s/grafana/'
    serve_from_sub_path: true

otherwise if not using port

grafana.ini:
  server:
    root_url: '%(protocol)s://%(domain)s:/grafana/'
    serve_from_sub_path: true

so the grafana.url should be below

linkerd viz install --set grafana.url=grafana.grafana \
  | kubectl apply -f -

doesn't need port

[Paarth Agarwal]

I'd like to work on this issue