Change deprecated kUUTypeUTF8PlainText for UTType

[iamgabrielma]

Description

By updating the minimum release version to iOS 15 recently, we've seen some deprecation warnings. This PR fixes the case for uses of kUUTypeUTF8PlainText

Changes

Changed the two references of kUUTypeUTF8PlainText for UTType.utf8PlainText.identifier, in the ProductDownloadDragAndDrop class.

This type is used when we need to represent plain text encoded as UTF-8, this is needed in this case when we attempt to drag and drop elements from our table view. The system will call the writableTypeIdentifiersForItemProvider and readableTypeIdentifiersForItemProvider methods to get information about our encoded data representation while dragging and dropping those elements around.

Testing instructions

• Create or edit a "Virtual and Downloadable" product from wp-admin
• Add several downloadable files.
• Go the app and navigate to the "Downloadable Files" view
• Try to drag and drop the cells to reorder the files, this should work as expected

Screenshots

Downloadable files	Drag and drop

[wpmobilebot]

You can test the changes from this Pull Request by:

• Clicking here or scanning the QR code below to access App Center
• Then installing the build number pr8188-d681ea8 on your iPhone

If you need access to App Center, please ask a maintainer to add you.

[toupper]

Nice! Fewer warnings 🎉 Code is good and tests well