Ensure that dashboard files are named correctly, e.g. end with .json

This fixes the previous commit and fixes the last issue I had to work
out with this role, dashboards.  Dashboards can now be added using the
grafana_server_dashboads dictionary.

tasks/dashboards.yml

@@ -24,7 +24,7 @@
 - name: Create dashboard JSON files
   ansible.builtin.copy:
     content: "{{ item.value.json }}"
-    dest: "{{ grafana_server_paths_dashboards }}/{{ item.value.folder | default('') }}/{{ item.key }}"
+    dest: "{{ grafana_server_paths_dashboards }}/{{ item.value.folder | default('') }}/{{ item.key }}.json"
     owner: root
     group: grafana
     mode: "0640"