Added Exceptiongroup handling to migration guide

Author: antonpirker

MIGRATION_GUIDE.md

@@ -22,6 +22,7 @@ Looking to upgrade from Sentry SDK 2.x to 3.x? Here's a comprehensive list of wh
 - clickhouse-driver integration: The query is now available under the `db.query.text` span attribute (only if `send_default_pii` is `True`).
 - `sentry_sdk.init` now returns `None` instead of a context manager.
 - The `sampling_context` argument of `traces_sampler` and `profiles_sampler` now additionally contains all span attributes known at span start.
+- We updated how we handle `ExceptionGroup`s. You will now get more data if ExceptionGroups are appearing in chained exceptions.
 - The integration-specific content of the `sampling_context` argument of `traces_sampler` and `profiles_sampler` now looks different.
   - The Celery integration doesn't add the `celery_job` dictionary anymore. Instead, the individual keys are now available as: