Remove support for Webpack 3.x and prior from `WatchTimestampsPlugin`.
Remove support for Webpack 3.x and prior from `WatchTimestampsPlugin`.

GoogleChromeLabs/squoosh,GoogleChromeLabs/squoosh,GoogleChromeLabs/squoosh,GoogleChromeLabs/squoosh,GoogleChromeLabs/squoosh,GoogleChromeLabs/squoosh