[BACKPORT][v1.6.1][BUG] Volumes stuck upgrading after 1.5.3 -> 1.6.0 upgrade.

[github-actions]

backport #7887

[longhorn-io-github-bot]

Pre Ready-For-Testing Checklist

• Where is the reproduce steps/test steps documented?
  So far we cannot reproduce, though making these exact changes in the user's cluster got Longhorn working again. I suggest we just verify the CI tests still pass after the PRs merge.

• Is there a workaround for the issue? If so, where is it documented?
  The workaround is at: [BUG] Volumes stuck upgrading after 1.5.3 -> 1.6.0 upgrade. #7887 (comment).

• Does the PR include the explanation for the fix or the feature?

• Does the PR include deployment change (YAML/Chart)? If so, where are the PRs for both YAML file and Chart?
  The PR for the YAML change is at: Make InstanceProcessStatus.Conditions nullable (backport #7898) #7900.
  The PR for the chart change is at: Make InstanceProcessStatus.Conditions nullable (backport #7898) #7900.
  Another PR is at: Explicitly set preserveUnknownFields to false (backport #7916) #7953.

• Have the backend code been merged (Manager, Engine, Instance Manager, BackupStore etc) (including backport-needed/*)?
  The PR is at: Make InstanceProcessStatus.Conditions nullable (backport #2574) longhorn-manager#2575.
  Another PR is at: Explicitly set preserveUnknownFields to false (backport #2580) longhorn-manager#2587.

[roger-ryao]

Verified on v1.6.x-head 20240301

• longhorn v1.6.x-head 3382579
• longhorn-manager v1.6.x-head longhorn/longhorn-manager@654ee92

Result Passed

Upgrade from v1.5.4 to v1.6.x-head:
https://ci.longhorn.io/job/private/job/longhorn-tests-regression/6578/

Upgrade from v1.4.4 to v1.5.4 to v1.6.x-head:
https://ci.longhorn.io/job/private/job/longhorn-tests-regression/6579/