make pushNotificationServer configurable in E2E (#25929)

e2e-tests/cypress/cypress.config.ts

@@ -36,6 +36,7 @@ export default defineConfig({
         minioS3Endpoint: 'localhost:9000',
         minioS3SSL: false,
         numberOfTrialUsers: 100,
+        pushNotificationServer: 'https://push-test.mattermost.com',
         resetBeforeTest: false,
         runLDAPSync: true,
         secondServerURL: 'http://localhost/s/p',