Add deprecation note about the unofficial first version of backend plugins (#24675)

pkg/plugins/backendplugin/backend_plugin.go

@@ -98,6 +98,8 @@ func (p *BackendPlugin) start(ctx context.Context) error {
 			}
 		}
 	} else {
+		p.logger.Warn("Plugin uses a deprecated version of Grafana's backend plugin system which will be removed in a future release. " +
+			"Consider upgrading to a newer plugin version or reach out to the plugin repository/developer and request an upgrade.")
 		raw, err := rpcClient.Dispense(p.id)
 		if err != nil {
 			return err