azure: document migration to new GPU Node label

[nojnhuh]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

This PR documents the Azure provider's plan to migrate to a new label used to identify GPU Nodes in order to align with the labels used by AKS: https://learn.microsoft.com/en-us/azure/aks/use-labels#deprecated-labels

The rough plan is to:

• document that users need to set BOTH the current and new labels on GPU Nodes (this PR)
• wait for at least one minor version release cycle to allow users time to add the new label (until at least cluster-autoscaler 1.32)
• change the Azure provider from identifying GPU Nodes based on the old label to the new one

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

ACTION REQUIRED: VMSS GPU Nodes must now also include the `kubernetes.azure.com/accelerator` label in addition to `accelerator`.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[nojnhuh]

/assign @comtalyst

[comtalyst]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: comtalyst, nojnhuh

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• cluster-autoscaler/cloudprovider/azure/OWNERS [comtalyst]

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