OSsku in-place migration on AKS

[pokushwaha]

The OSsku in-place migration feature allows the user to trigger a node image upgrade between one Linux SKU (i.e. Ubuntu) to another (i.e. Azure Linux) on an existing nodepool. Today, traditional OSsku migration involves creating a new node, cordoning and draining existing nodes, and then deleting existing nodes. This can involve a large surge of core count as new nodes are added, as well as manual intervention to cordon and drain.

This feature is available for public preview and can be deployed via ARM template and CLI (via AKS preview extension). Once GA, it will be available via Terraform as well.

Link to docs: https://learn.microsoft.com/en-us/azure/azure-linux/tutorial-azure-linux-migration?tabs=azure-cli#in-place-os-sku-migration-preview