[IMPROVEMENT] Reject the last replica deletion if its volume.spec.deletionTimestamp is not set

[derekbit]

Is your improvement request related to a feature? Please describe (👍 if you like this request)

To prevent accidentally from deleting the last replica regardless of it is healthy or faulty, reject the last replica deletion if spec.deletionTimestamp of its volume is not set in the validating webhook.

Describe the solution you'd like

Describe alternatives you've considered

Additional context

[longhorn-io-github-bot]

Pre Ready-For-Testing Checklist

• Where is the reproduce steps/test steps documented?
  The reproduce steps/test steps are at:

Create a volume with 3 replicas.
Delete first replica and second replica. When deleting the third replica, the deletion should be rejected.

• 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:
  The PR for the chart change is at:

• Have the backend code been merged (Manager, Engine, Instance Manager, BackupStore etc) (including backport-needed/*)?
  The PR is at

longhorn/longhorn-manager#2395

• Which areas/issues this PR might have potential impacts on?
  Area: volume, replica
  Issues

[yangchiu]

Verified passed on master-head (longhorn-manager 859d438) following test steps. Try to remove the last replica, error message shown:

failed to remove replica: no other healthy replica available, cannot delete replica test-1-r-b9d4c389 since it may still contain data for recovery