Update release notes

Author: itsmeichigo

RELEASE-NOTES.txt

@@ -7,6 +7,7 @@
 - [*] Improved image loading in Blaze Campaign Creation: displays a redacted and shimmering effects when loading product image and falls back to a placeholder if no image is available. [https://github.com/woocommerce/woocommerce-ios/pull/15098]
 - [*] Product List: Display syncing animation on items with image upload in progress [https://github.com/woocommerce/woocommerce-ios/pull/15052]
 - [*] Background image upload: Fix missing error notice in iPhones [https://github.com/woocommerce/woocommerce-ios/pull/15117]
+- [*] Background image upload: Show a notice when the user leaves product details while uploads are pending [https://github.com/woocommerce/woocommerce-ios/pull/15134]
 
 21.7
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