Release Notes: add new section for next version (21.4)

Author: wpmobilebot

RELEASE-NOTES.txt

@@ -1,5 +1,9 @@
 *** PLEASE FOLLOW THIS FORMAT: [<priority indicator, more stars = higher priority>] <description> [<PR URL>]
 *** Use [*****] to indicate smoke tests of all critical flows should be run on the final IPA before release (e.g. major library or OS update).
+21.4
+-----
+
+
 21.3
 -----
 - [*] Jetpack Setup: Fixed an issue where the WordPress.com authentication fails when using a passwordless account that's already connected to Jetpack [https://github.com/woocommerce/woocommerce-ios/pull/14501]