[IMPROVEMENT] Adding a message to hint users to clean up non-existing disks in Backing Image CR

[PhanLe1010]

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

Sometime backing image CR in this cluster are referring non-existing disks which might never come back. Looks like these disks are orphan. I don't see we have logic to clean up these disk in BackingImage CR.
2025-02-24T01:38:35.833470677Z time="2025-02-24T01:38:35Z" level=warning msg="Disk is not ready hence backing image manager can not be created" func="controller.(*BackingImageController).handleBackingImageManagers" file="backing_image_controller.go:738" backingImageName=default-image-vqk8h controller=longhorn-backing-image diskUUID=f5fdea54-a6de-4cd5-9627-29de0a515c0d error="cannot find the corresponding ready node and disk with disk UUID f5fdea54-a6de-4cd5-9627-29de0a515c0d" node=oml-harvester-18

Describe the solution you'd like

Idea from @ChanYiLin

I think the reason is that we dont know if the disk/node is gone forever or not
If we clean up the disk from the bi spec
Then when the disk/node is back, longhorn will delete the file on the disk/node
We can probably add a warning message to let user decide if they want to delete that disk from the spec
They can delete the disk from the spec by click “clean up” in the backing image page in the “operation” for each disk

Describe alternatives you've considered

No response

Additional context

No response