add deprecation warning

Author: mydea

docs/migration/draft-v9-migration-guide.md

@@ -87,6 +87,10 @@
   });
   ```
 
+## `@sentry/opentelemetry`
+
+- Deprecated `generateSpanContextForPropagationContext` in favor of doing this manually - we do not need this export anymore.
+
 ## Server-side SDKs (`@sentry/node` and all dependents)
 
 - Deprecated `processThreadBreadcrumbIntegration` in favor of `childProcessIntegration`. Functionally they are the same.