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

CHANGELOG.md

@@ -13,6 +13,7 @@
 **Deprecation warnings**
 
 - Scripted dashboards is now deprecated. The feature is not removed but will be in a future release. We hope to address the underlying requirement of dynamic dashboards in a different way. [#24059](https://github.com/grafana/grafana/issues/24059)
+- The unofficial first version of backend plugins together with usage of [grafana/grafana-plugin-model](https://github.com/grafana/grafana-plugin-model) is now deprecated and support for that will be removed in a future release. Please refer to [backend plugins documentation](https://grafana.com/docs/grafana/latest/developers/plugins/backend/) for information about the new officially supported backend plugins.
 
 ## 7.0 Feature highlights
 

docs/sources/menu.yaml

@@ -407,8 +407,6 @@
               name: Defaults and editor mode
             - link: /developers/plugins/legacy/apps/
               name: App plugins
-            - link: /developers/plugins/legacy/panels/
-              name: Backend plugins
             - link: /developers/plugins/legacy/data-sources/
               name: Data source plugins
             - link: /developers/plugins/legacy/snapshot-mode/

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