Promote dual-stack to Beta

[kfaseela]

Reference: istio/enhancements#196

Reviewers

• Ambient
• Docs
• [] Installation
• Networking
• Performance and Scalability
• Extensions and Telemetry
• Security
• Test and Release
• User Experience
• Developer Infrastructure
• Localization/Translation

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[dhawton]

/test lint

[kfaseela]

/retest