Docs: Replace sync check usage with async check

[andrewslotin]

Change all, from:

const headerText = await page.locator('h2').textContent();
check(headerText, {
  'header': headerText === 'Welcome, admin!'
});

To:

import { check } from 'https://jslib.k6.io/k6-utils/1.5.0/index.js';

// ...

check(page.locator('h2'), {
  'header': async lo => await lo.textContent() === 'Welcome, admin!'
});

Tasks

Preview Give feedback

1. Use async check instead of the sync one #1746
   Area: browser +1
   
2. Update sync examples with check to async check xk6-browser#1448
   ci documentation +2
   

[inancgumus]

We'll also update the xk6-browser examples/.

Done: grafana/xk6-browser#1448