Update functions changelog (#6751)

The mergeback PR from last release didn't include the update to
function's CHANGELOG.md file

Author: rlazo

firebase-functions/CHANGELOG.md

@@ -1,8 +1,15 @@
 # Unreleased
+
+# 21.1.1
 * [fixed] Resolve Kotlin migration visibility issues
   ([#6522](//github.com/firebase/firebase-android-sdk/pull/6522))
 
 
+## Kotlin
+The Kotlin extensions library transitively includes the updated
+`firebase-functions` library. The Kotlin extensions library has no additional
+updates.
+
 # 21.1.0
 * [changed] Migrated to Kotlin