meta: Backfill v9 migration guide items

Author: Luca Forstner

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

@@ -2,6 +2,10 @@
 
 # Deprecations
 
+## General
+
+- Returning `null` from `beforeSendSpan` span is deprecated.
+
 ## `@sentry/utils`
 
 - **The `@sentry/utils` package has been deprecated. Import everything from `@sentry/core` instead.**
@@ -17,6 +21,7 @@
 - Deprecated `memoBuilder`. No replacements.
 - Deprecated `BAGGAGE_HEADER_NAME`. No replacements.
 - Deprecated `makeFifoCache`. No replacements.
+- Deprecated `flatten`. No replacements.
 
 ## `@sentry/core`