Revert "Pin version of grafana-oss container"

This reverts commit c5ca06370c.

This gets us back to being on the latest grafana images after the
problems from the beta release [1].  Upstream is already on to a 9.1.1
release -- it's better if we don't fall behind as it just makes it
even harder to catch up later.

[1] https://github.com/grafana/grafana/issues/53275

Change-Id: I8bfe7b31233243f47e292a45a85e718a804d75bb

playbooks/roles/grafana/templates/docker-compose.yaml.j2

@@ -5,7 +5,7 @@ version: '2'
 services:
   grafana:
     restart: always
-    image: docker.io/grafana/grafana-oss:9.0.4
+    image: docker.io/grafana/grafana-oss:latest
     network_mode: host
     environment:
       GF_SERVER_DOMAIN: 'grafana.opendev.org'