Update Pod CIDR recommendation with RFC 6598

[tyler-lloyd]

Added reference to RFC 6598 for Pod CIDR private ranges.

Addresses Azure/AKS#5299

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[v-dirichards]

@asudbring

Can you review the proposed changes?

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[Copilot]

Pull Request Overview

Adds an additional standards reference (RFC 6598) to the documentation of recommended Pod CIDR ranges.

• Expanded list of CIDR standards from only RFC 1918 to also include RFC 6598.
• Modification occurs in an [!IMPORTANT] advisory block within Azure CNI Overlay guidance.

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[Copilot]

RFC 6598 (100.64.0.0/10) defines Shared Address Space for carrier-grade NAT, not private address space like RFC 1918; labeling it as a 'private CIDR range' is inaccurate and may mislead users. Suggest rewording to: 'The address spaces suitable for Pod CIDR include the private ranges defined in RFC 1918 and, where appropriate, the shared address space (RFC 6598, 100.64.0.0/10). Note that RFC 6598 is reserved for carrier-grade NAT and its use may have operational implications.'

Suggested change

> The private CIDR ranges available for the Pod CIDR are defined in [RFC 1918](https://tools.ietf.org/html/rfc1918) and [RFC 6598](https://tools.ietf.org/html/rfc6598). While we don't block the use of public IP ranges, they are considered out of Microsoft's support scope. We recommend using private IP ranges for pod CIDR.

> The address spaces suitable for Pod CIDR include the private ranges defined in [RFC 1918](https://tools.ietf.org/html/rfc1918) and, where appropriate, the shared address space ([RFC 6598](https://tools.ietf.org/html/rfc6598), 100.64.0.0/10). Note that RFC 6598 is reserved for carrier-grade NAT and its use may have operational implications. While we don't block the use of public IP ranges, they are considered out of Microsoft's support scope. We recommend using private IP ranges for pod CIDR.