Fix incorrect "stopped impersonation" log message

Closes #21015

app/controllers/admin/impersonations_controller.rb

@@ -7,7 +7,7 @@ class Admin::ImpersonationsController < Admin::ApplicationController
     warden.set_user(impersonator, scope: :user)
-    Gitlab::AppLogger.info("User #{original_user.username} has stopped impersonating #{impersonator.username}")
+    Gitlab::AppLogger.info("User #{impersonator.username} has stopped impersonating #{original_user.username}")
     session[:impersonator_id] = nil

spec/controllers/admin/impersonations_controller_spec.rb

@@ -77,6 +77,8 @@ describe Admin::ImpersonationsController do
           context "when the impersonator is not blocked" do
             it "redirects to the impersonated user's page" do
+              expect(Gitlab::AppLogger).to receive(:info).with("User #{impersonator.username} has stopped impersonating #{user.username}").and_call_original
+
               delete :destroy
               expect(response).to redirect_to(admin_user_path(user))