Add migration note about SentrySpanProcessor and SentryPropagator

Author: sl0thentr0py

MIGRATION_GUIDE.md

@@ -27,6 +27,7 @@ Looking to upgrade from Sentry SDK 2.x to 3.x? Here's a comprehensive list of wh
 - 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. It could happen that after updating the SDK the grouping of issues change because of this. So eventually you will see the same exception in two Sentry issues (one from before the update, one from after the update)
 - The integration for Python `logging` module does not send Sentry issues by default anymore when calling `logging.error()`, `logging.critical()` or `logging.exception()`. If you want to preserve the old behavior use `sentry_sdk.init(integrations=[LoggingIntegration(event_level="ERROR")])`.
+- The `SentrySpanProcessor` and `SentryPropagator` are exported from `sentry_sdk.opentelemetry` instead of `sentry_sdk.integrations.opentelemetry`.
 - 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: