invalidate pages when release is yanked

[jsha]

Part of #1552, filing as its own issue so we don't forget:

When a crate version is yanked, we should create an invalidation for that crate.

Note: I looked to see how / when we get information about yanks. It looks like it happens in index::Api::get_release_data. But that appears to only get called during builds. Is that right?

[Nemo157]

Yanks get detected by the "registry watcher" here:

docs.rs/src/build_queue.rs

Line 264 in 3e4c205

Change::Yanked(release) => {

[syphar]

This is definitely a gap in the current logic, probably + a crate-delete event.

I would fix this after we merged #1856, but before we introduce the default TTL ( = before we actually start caching)