Replace first_meaningful_paint with largest_contentful_paint

[mdcruz]

According to https://web.dev/first-meaningful-paint/:

First Meaningful Paint (FMP) is deprecated in Lighthouse 6.0. In practice FMP has been overly sensitive to small differences in the page load, leading to inconsistent (bimodal) results. Additionally, the metric's definition relies on browser-specific implementation details, which means it cannot be standardized nor implemented in all web browsers. Moving forward, consider using Largest Contentful Paint instead.

We should aim to replace first_meaningful_paint metric with largest_contentful_paint or LCP instead. LCP metric is being tracked on the Performance tab of Chrome. I think this isn’t part of DevTools API though but it’s part of the browser web/standards API