[release/1.7] snapshots: emit deprecation warning for aufs

[samuelkarp]

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.

[samuelkarp]

/retest

[samuelkarp]

/cherrypick release/1.6

[k8s-infra-cherrypick-robot]

@samuelkarp: #9436 failed to apply on top of branch "release/1.6":

Applying: snapshots: emit deprecation warning for aufs
Using index info to reconstruct a base tree...
M	pkg/deprecation/deprecation.go
M	services/snapshots/service.go
Falling back to patching base and 3-way merge...
Auto-merging services/snapshots/service.go
CONFLICT (content): Merge conflict in services/snapshots/service.go
Auto-merging pkg/deprecation/deprecation.go
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 snapshots: emit deprecation warning for aufs
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

In response to this:

/cherrypick release/1.6

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