fix: attempt to delete processes from instance-managers in unknown state (backport #3127)

[mergify]

Which issue(s) this PR fixes:

longhorn/longhorn#6552

What this PR does / why we need it:

See longhorn/longhorn#6552 (comment) for context. It may be possible to delete engine and replica processes from an instance-manager even when the instance-manager's state is unknown. Doing so prevents the processes from becoming orphans if the instance-manager eventually recovers from the unknown state.

This PR is looking good in some local testing, but I want to put it through a few more paces before review.

---

This is an automatic backport of pull request #3127 done by Mergify.

[mergify]

Cherry-pick of 8629fd7 has failed:

On branch mergify/bp/v1.5.x/pr-3127
Your branch is up to date with 'origin/v1.5.x'.

You are currently cherry-picking commit 8629fd72.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   controller/engine_controller.go

no changes added to commit (use "git add" and/or "git commit -a")

Cherry-pick of 72e2445 has failed:

On branch mergify/bp/v1.5.x/pr-3127
Your branch is ahead of 'origin/v1.5.x' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 72e24457.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   controller/engine_controller.go
	modified:   engineapi/instance_manager.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   controller/instance_manager_controller.go
	both modified:   controller/replica_controller.go

Cherry-pick of 1a1ab36 has failed:

On branch mergify/bp/v1.5.x/pr-3127
Your branch is ahead of 'origin/v1.5.x' by 2 commits.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 1a1ab369.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   controller/replica_controller.go

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally