[11.x] Unify prefetch API

[timacdonald]

Unify the prefetching API.

// Waterfall with 3 active requests...
Vite::prefetch(concurrency: 3);

// Everything all at once...
Vite::prefetch();

[gianfranco-rocco]

Suggested change

	? $this->usePrefetchStrategy('aggressive')
	: $this->usePrefetchStrategy('waterfall', ['concurrency' => $concurrency]);
	? $this->useAggressivePrefetching()
	: $this->useWaterfallPrefetching($concurrency);

While this works, it would've been a good idea to use these methods the class already provides.