Document boto3 span naming change in 3.0 migration guide #8207

Add the boto3 changes in 3.0 to the docs page 2.x to 3.x migration guide (docs/platforms/python/migration/2.x-to-3.x.mdx in sentry-docs).

• The docs page does not mention boto3 today.
• The SDK MIGRATION_GUIDE.md on major/3.0 already has the text. Use it as the source:
  • Span names are now <ServiceID>.<Operation> (for example S3.GetObject), not aws.<service-id>.<Operation>. The origin is now auto.aws.boto3.
  • AWS calls use the rpc op, not http.client.
• After M1 merges: add to both guides that SQS and SNS messaging calls use queue.* ops.
• Agent drafts the change. Johannes reviews.

Action taken on behalf of Johannes Daxböck.