[Paper] Update the paper Gallery app to 0.81.0-preview.6

Refer this PR and update the paper app to 0.81.0-preview.6 https://github.com/microsoft/react-native-gallery/pull/571/files
You would need to update the package.json as listed in above PR.
The diff should be similar.

update minor version here windows/rngallery/Package.appxmanifest increment by 1

update comment in this file to point to latest current version windows/rngallery/rngallery.vcxproj