API: Fix error code for disabled popular endpoint (#4296)

When visiting /api/v1/popular and popular endpoint is disabled
Before:

500 {"error":"Closed stream"}

After

403 {"error":"Administrator has disabled this endpoint."}

src/invidious/routes/api/v1/feeds.cr

@@ -31,7 +31,7 @@ module Invidious::Routes::API::V1::Feeds
 
     if !CONFIG.popular_enabled
       error_message = {"error" => "Administrator has disabled this endpoint."}.to_json
-      haltf env, 400, error_message
+      haltf env, 403, error_message
     end
 
     JSON.build do |json|