docs: remove outdated instruction

[JoeyC-Dev]

• Basic SKU is no longer supported as of September 30, 2025.
• Azure workload identity is in GA: [Feature] Azure AD Workload Identity Managed AKS Addon Azure/AKS#1480 (comment). Hence, aks-preview is no longer required.

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[cert-manager-prow]

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[JoeyC-Dev]

/assign cm-maintainers
/unassign thatsmrtalbot

[cert-manager-prow]

@JoeyC-Dev: GitHub didn't allow me to assign the following users: cm-maintainers.

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[JoeyC-Dev]

/assign hawksight