Don't start invalid but enabled plugins at startup

For example during the startup after RabbitMQ was upgraded but an
enabled community plugin wasn't, and the plugin's broker version
requirement isn't met any more, RabbitMQ still started the plugin
after logging an error.

deps/rabbit/src/rabbit_plugins.erl

@@ -265,7 +265,7 @@ prepare_plugins(Enabled) ->
                                       [ExpandDir, E2]}})
     end,
     [prepare_plugin(Plugin, ExpandDir) || Plugin <- ValidPlugins],
-    Wanted.
+    [P#plugin.name || P <- ValidPlugins].
 
 maybe_warn_about_invalid_plugins([]) ->
     ok;