Switch deprecations audit from ChromeConsoleMessages => ReportingObserver

The current impl uses the `ChromeConsoleMessages` gatherer and [filters on](https://github.com/GoogleChrome/lighthouse/blob/master/lighthouse-core/audits/deprecations.js#L39-L46) `message.type === 'deprecation'`.

This works great, but [`ReportingObserver`](https://www.chromestatus.com/feature/4691191559880704) provides a better structured API intended for observing deprecations. It has other nice feature like buffering (can include deprecations that happen before the observer was added to the page) useful fields like `anticipatedRemoval`:

Switching to a `ReportingObserver`-based would set us up nicely for future audits that use the Reporting API: CSP violations, feature policy violations, browser interventions.