Switch deprecations audit from ChromeConsoleMessages => ReportingObserver

[ebidel]

The current impl uses the ChromeConsoleMessages gatherer and filters on message.type === 'deprecation'.

This works great, but ReportingObserver 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.

[brendankenny]

Ideally we could do this through the protocol as inserting observers into the host page has a host of problems.