Release Notes: add new section for next version (21.7)

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 APK before release (e.g. major library or targetSdk updates).
 *** For entries which are touching the Android Wear app's, start entry with `[WEAR]` too.
+21.7
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+
+
 21.6
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 - [*] Updated tablet detection logic to consider both width and height dimensions, enhancing compatibility with larger phones and tablets, particularly in landscape mode [https://github.com/woocommerce/woocommerce-android/pull/13228]