Make creation of imported clusters use v3/management cluster instead of v1/provisioning

[eva-vashkevich]

Is your feature request related to a problem? Please describe.
There is an inconsistency between POST operations for imported cluster. We want to make it more consistent and avoid adding non-provisioning fields or duplicating fields from the v3 cluster to the v1 cluster.

Describe the solution you'd like

We want to switch to using v3/management cluster for both POST operations (creation and edit, currently it is only used for edit).

[richard-cox]

This will hopefully be a short term step until the next phase of cluster management comes in.

It also aligns with how the cluster will be edited (port from ember v3/cluster to v1/mgmt cluster - #13064)

[richard-cox]

Linking rancher/rancher#35192