metricspi: Fix vmalert links

1. Grafana 8 changed the format of the query string parameters for the
   Explore page.
2. vmalert no longer needs the http.pathPrefix argument when behind a
   reverse proxy, rather it uses the request path like the other
   Victoria Metrics components.

group_vars/metricspi/vmalert.yml

@@ -5,6 +5,5 @@ vmalert_remote_write_url: http://[::1]:8428
 vmalert_external_url: https://grafana.pyrocufflink.blue
 vmalert_external_alert_source: >-
   {% raw -%}
-  explore?orgId=1&left=%5B"now-1h","now","metrics",%7B"expr":"{{$expr|queryEscape}}"%7D%5D
+  explore?orgId=1&left=%7B"queries":%5B%7B"expr":"{{$expr|queryEscape}}"%7D%5D%7D
   {%- endraw %}
-vmalert_http_path_prefix: /vmalert/