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

Some changes for a better user experience (only user-facing part, we don't need to change anything internally). Also, for all changes below, need to update the doc accordingly if needed.

Describe the solution you'd like

• Use v1 for the current engine, and v2 for the SPDK-based engine instead of Longhorn vs SPDK
• Change DO NOT CHANGE THIS SETTING WITH ATTACHED VOLUMES. Longhorn will try to block this setting update when there are attached volumes. Remove try, just said Longhorn will block ...
• Add a known issue about the actual size unable to display in the doc. Also, need a corresponding issue for later SPDK part improvement.
  • tracked in [IMPROVEMENT] Expose actual size of a logical volume #5947
• Change replica number to 3 https://github.com/longhorn/longhorn/blob/master/examples/spdk/storageclass.yaml#L11, also the original storage class example for the old stack.
• We already highlight CPU usage for SPDK in doc but suggest emphasizing it to the setting as well to notice users actively.

Describe alternatives you've considered

A clear and concise description of any alternative solutions or features you've considered.

Additional context

cc @longhorn/dev-data-plane