docs: add guidance on migrating between services without downtime

[Thiru-2004-varul]

Issue

Fixes #4810

Description

Adds a short section to the blue/green traffic splitting doc explaining how to migrate an Ingress backend from one Service to another without dropping active connections.

Currently, directly changing the backend service name causes the controller to delete the existing target group and create a new one, dropping connections immediately without respecting deregistration_delay.timeout_seconds. This isn't a bug — it's existing behavior — but the recommended alternative (weighted target groups via the actions.* annotation) wasn't documented for this specific use case.

This PR adds a "Migrating between services without downtime" section to docs/guide/use_cases/blue_green/index.md, describing how to use the existing weighted target group feature to shift traffic gradually and avoid dropped connections.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: Thiru-2004-varul / name: Thiru-2004-varul (34dc451)

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[kubernetes-prow]

Hi @Thiru-2004-varul. Thanks for your PR.

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[shraddhabang]

/lgtm
Thanks for the changes.

[kubernetes-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: shraddhabang, Thiru-2004-varul

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