Use grafana_dashboard_template_files variable according to galaxy/ansible-role-grafana changes

group_vars/matrix_servers

@@ -5421,7 +5421,7 @@ grafana_dashboard_download_urls: |
     (matrix_synapse_usage_exporter_dashboard_urls if matrix_synapse_usage_exporter_enabled else [])
   }}
 
-grafana_provisioning_dashboard_template_files: |
+grafana_dashboard_template_files: |
   {{
     ([{
         'path': 'roles/custom/matrix-prometheus-nginxlog-exporter/templates/grafana/nginx-proxy.json',