Add AKS Fleet update strategy quickstart

[Rebecca Calixte (Rebecca-Calixte)]

Adds a Terraform quickstart for Azure Kubernetes Fleet Manager reusable update strategies, migrated from the local sample created for ADO 567237.

Summary:

• Creates quickstart/201-aks-fleet-update-strategy
• Preserves the AzureRM-native azurerm_kubernetes_fleet_update_strategy implementation
• Preserves the staged update behavior from the source sample: stage-1 with group-1 and group-2, after_stage_wait_in_seconds = 300, followed by stage-2 with group-3
• Excludes local execution artifacts such as .terraform, state files, plan files, and backups

Validation:

• terraform fmt -check -diff
• terraform init -backend=false -input=false
• terraform validate