Remove demo from readme as users can create free trial on production (#9952)

We have recently introduced the possibility to sign up on Twenty cloud
without having to input a credit card which makes the demo.twenty.com
useless. Deprecating it!

packages/twenty-e2e-testing/tests/demo/demo_basic.spec.ts

@@ -3,7 +3,7 @@ import { expect, test } from '@playwright/test';
 test('Check if demo account is working properly @demo-only', async ({
   page,
 }) => {
-  await page.goto('https://demo.twenty.com/');
+  await page.goto('https://app.twenty-next.com/');
   await page.getByRole('button', { name: 'Continue With Email' }).click();
   await page.getByRole('button', { name: 'Continue', exact: true }).click();
   await page.getByRole('button', { name: 'Sign in' }).click();