Backport of deployment watcher: fix goroutine leak when job is purged into release/1.5.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #20348 to be assessed for backporting due to the inclusion of the label backport/1.5.x.

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The below text is copied from the body of the original PR.

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The deployment watcher on the leader makes blocking queries to detect when the set of active deployments changes. It takes the resulting list of deployments and adds or removes watchers based on whether the deployment is active. But when a job is purged, the deployment will be deleted. This unblocks the query but the query result only shows the remaining deployments.

When the query unblocks, ensure that all active watchers have a corresponding deployment in state. If not, remove the watcher so that the goroutine stops.

Fixes: #19988

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Overview of commits

• a13e455

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