DEV: Replace current-user:main with service:current-user

This will allow consumers to inject it using `currentUser: service()` in preparation for the removal of implicit injections in Ember 4.0. `current-user:main` is still available and will print a deprecation notice.

app/assets/javascripts/discourse-common/addon/resolver.js

@@ -68,6 +68,11 @@ const DEPRECATED_MODULES = new Map(
       since: "2.9.0.beta7",
       dropFrom: "3.0.0",
     },
+    "current-user:main": {
+      newName: "service:current-user",
+      since: "2.9.0.beta7",
+      dropFrom: "3.0.0",
+    },
   })
 );