Update Migration Assistant docs for OpenSearch Serverless NextGen

[jugal-chauhan]

Description

Updates the Migration Assistant section to reflect Amazon OpenSearch Serverless NextGen GA.

NextGen is the new default architecture for Amazon OpenSearch Serverless. Existing docs were written for the Classic architecture only and don't mention collection groups, the new endpoint format, or the inheritance model. This PR pivots the Serverless content across the Migration Assistant tree so users following the playbooks land on a working NextGen migration path.

Key changes

• Added "Amazon OpenSearch Serverless NextGen" to Vale accept texts.
• Renamed all references from Amazon OpenSearch Serverless to Amazon OpenSearch Serverless NextGen across all migration-assistant pages (full form on first use per page, then Serverless NextGen for brevity).
• Updated the example collection endpoint format from <id>.<region>.aoss.amazonaws.com to <id>.aoss.<region>.on.aws (the NextGen format).
• Added a new "Create the collection group" prerequisite step to the AOS-to-AOSS playbook, since collection groups are required for NextGen.
• Updated the create-collection command in that playbook to include --collection-group-name. Generation is inherited from the parent collection group, so no --generation flag on create-collection.

URL preservation

Permalinks (/migration-assistant/amazon-opensearch-serverless/ and /migration-assistant/playbook-amazon-opensearch-service-to-serverless/) are unchanged so existing external links keep resolving. Only link text and page titles have been updated.

Testing

The end-to-end migration path described in the updated playbooks was validated on a live NextGen SEARCH collection - a sample Apache Solr 8.11 source was successfully backfilled into a NextGen collection using Migration Assistant, including collection-group creation, metadata translation, and document indexing.

Issues Resolved

N/A

Version

N/A

Frontend features

If you're submitting documentation for an OpenSearch Dashboards feature, add a video that shows how a user will interact with the UI step by step. A voiceover is optional.

Checklist

• By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and subject to the Developers Certificate of Origin.
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[kolchfa-aws]

@jugal-chauhan Is this PR ready for doc review?