Release script: Update release notes for version 1.121.0

Author: Gerardo

RELEASE-NOTES.txt

@@ -1,5 +1,8 @@
 Unreleased
 ---
+
+1.121.0
+---
 * [internal] Fix Inserter items list filtering [https://github.com/wordpress-mobile/gutenberg-mobile/pull/6911]
 * [*] Prevent hiding the keyboard when creating new list items [https://github.com/wordpress-mobile/gutenberg-mobile/pull/6922]
 * [*] Fix issue when pasting HTML content [https://github.com/wordpress-mobile/gutenberg-mobile/pull/6933]