Part of #9312

aufs snapshotter is already removed from main, so this PR is directly against 1.7. However, the approach here is a bit weird, and I'm open to feedback on doing it differently if that's warranted. Here's the weirdness:

• aufs snapshotter is in a separate repo and currently depends on containerd 1.6.12
• The plugin itself has no dependencies
• Adding a dependency on the warning service would require:
  • an update to containerd 1.6 adding the new warning ID (and, preferably, a release)
  • updating the go module dependency in the aufs snapshotter
  • adding a dependency to the plugin
  • finally, updating the dependency in the containerd 1.6 and 1.7 release branches

So instead of that, I added the warning service as a dependency of the snapshot service, and emit the warning from there. It's certainly less elegant, but it was many fewer steps.