DEV: Install ember-this-fallback for plugins (#21236)

This will allow us to extend the deprecation period for this-property-fallback beyond Ember 4.x, to give more time for plugin developers to update their templates.

app/assets/javascripts/discourse-plugins/index.js

@@ -99,6 +99,10 @@ module.exports = {
     babel: {
       plugins: [require.resolve("deprecation-silencer")],
     },
+
+    "ember-this-fallback": {
+      enableLogging: false,
+    },
   },
 
   pluginInfos() {
@@ -243,11 +247,6 @@ module.exports = {
     return true;
   },
 
-  treeFor() {
-    // This addon doesn't contribute any 'real' trees to the app
-    return;
-  },
-
   // Matches logic from GlobalSetting.load_plugins? in the ruby app
   shouldLoadPlugins() {
     if (process.env.LOAD_PLUGINS === "1") {