Use matrix_grafana_enabled: true in the matrix-grafana role

This is consistent with what all other roles do. If someone includes a
role, the assumption is that they want its functionality enabled.

The playbook distribution then disables components via
`group_vars/matrix_servers`. We've always had `matrix_grafana_enabled: false`
there, so flipping the in-role `_enabled` flag to `true` does not change
anything for playbook users. Users who import the roles individually in
their own other playbooks (and who don't use `group_vars/matrix_servers`)
may observe a change in the defaults with this.

roles/matrix-grafana/defaults/main.yml

@@ -3,7 +3,7 @@
 # See: https://github.com/matrix-org/synapse/blob/master/docs/metrics-howto.md
 # Project source code URL: https://github.com/grafana/grafana
 
-matrix_grafana_enabled: false
+matrix_grafana_enabled: true
 
 matrix_grafana_version: 9.1.6
 matrix_grafana_docker_image: "{{ matrix_container_global_registry_prefix }}grafana/grafana:{{ matrix_grafana_version }}"