Preempted alloc during deploy guarantees failed deployment

### Nomad version
Output from `nomad version`
`Nomad v0.10.4 (f750636ca68e17dcd2445c1ab9c5a34f9ac69345)`

### Operating system and Environment details
Amazon Linux 2

### Issue
Hello, whenever a deploy is running a canary/bluegreen deploy, if an allocation is pre-empted before it reaches the healthy count, the entire deployment is doomed to fail by progress_deadline.

This is because the deployment will properly count the canary but when it pre-empts it, it does not replace it. So the final count would be N-1 running even though the deployment thinks all N are running. The deployment waits for that pre-empted allocation to get healthy, which of course, it never will since its no longer running, until the progress_deadline hits, then the red bar comes up and all the progress is rolled back.

This is very difficult to reproduce since I'm not too sure what causes a pre-empt action. But we do see it more often than not and is extremely problematic when it does happen.

Here is a deployment where that happened:

Here is the allocation that was redacted:

This may be related to an [earlier issue](https://github.com/hashicorp/nomad/issues/6407) I posted regarding deployment state falling out of sync that @tgross worked on a bit. Is it related? Should the pre-empted allocation have been detected and replaced by the deployment?

Thanks.

Start by tracing how Nomad handles deployments, canary or blue/green allocations, and allocation preemption, using the reported deployment and allocation screenshots as context. Reproduce or test the preemption case and verify that a preempted allocation is detected and replaced rather than leaving the deployment waiting until progress_deadline and rolling back.

Written by the indexing model from the issue text.