Replace obsolete HTTP status code :unprocessable_entity

The new naming is :unprocessable_content and required by Rack 3.1+

spec/controllers/code_ocean/files_controller_spec.rb

@@ -58,7 +58,7 @@ RSpec.describe CodeOcean::FilesController do
 
       expect_assigns(file: CodeOcean::File)
       expect_json
-      expect_http_status(:unprocessable_entity)
+      expect_http_status(:unprocessable_content)
     end
   end
 

spec/controllers/events_controller_spec.rb

@@ -29,13 +29,13 @@ RSpec.describe EventsController do
       before { post :create, params: {event: {exercise_id: 847_482}} }
 
       expect_assigns(event: Event)
-      expect_http_status(:unprocessable_entity)
+      expect_http_status(:unprocessable_content)
     end
 
     context 'with no event' do
       before { post :create }
 
-      expect_http_status(:unprocessable_entity)
+      expect_http_status(:unprocessable_content)
     end
   end
 end

spec/controllers/submissions_controller_spec.rb

@@ -36,7 +36,7 @@ RSpec.describe SubmissionsController do
 
         expect_assigns(submission: Submission)
         expect_json
-        expect_http_status(:unprocessable_entity)
+        expect_http_status(:unprocessable_content)
       end
     end