Release Notes: add new section for next version (23.9)

Author: wpmobilebot

RELEASE-NOTES.txt

@@ -1,6 +1,10 @@
 *** 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).
 
+23.9
+-----
+
+
 23.8
 -----
 - [*] Better support site credential login for sites with captcha plugins [https://github.com/woocommerce/woocommerce-ios/pull/16372]