Move release notes entry for #20275 to version 21.8.1

The fix in #20275 was originally made on top of the `release/21.9`
branch but we decided to ship it as a hotfix.

I could have amended 9dfe337 with this
change, but decided to keep it in one piece to preserve a record of the
original change.

Author: mokagio

RELEASE-NOTES.txt

@@ -6,6 +6,10 @@
 * [**] [Jetpack-only] Jetpack individual plugin support: Warns user about sites with only individual plugins not supporting all features of the app yet and gives the ability to install the full Jetpack plugin. [#20223]
 * [**] [Jetpack-only] Help: Display the Jetpack app FAQ card on Help screen when switching from the WordPress app to the Jetpack app is complete. [#20232]
 * [***] [Jetpack-only] Blaze: We added support for Blaze in the app. The user can now promote a post or page from the app to reach new audiences. [#20253]
+
+21.8.1
+-----
+
 * [**] [internal] Fixes a crash that happens in the background when the weekly roundup notification is being processed. [#20275]
 
 21.8