Updating ARO Quickstart with MIWI notes

[diana-sari]

Summary

This update refreshes the ARO Quickstart to distinguish between the two supported credential models.

Key changes include:

• Recommends managed identities with platform workload identity federation for new ARO clusters.
• Adds the Azure CLI 2.84.0 requirement for managed-identity deployments.
• Links to the dedicated MIWI deployment guide.
• Clearly labels the existing deployment steps as the service-principal path.
• Warns that service-principal clusters cannot be converted in place to managed identities.
• Adds cleanup guidance specific to managed-identity clusters.

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