Harmonize release notes

[asbjornu]

As I think #21100 reveals, there's some confusion around how Fastlane deals with changelogs or release notes for different release targets.

iOS

For TestFlight, upload_to_testflight has a :changelog option. For App Store releases you can use either set_changelog or upload_to_app_store with the :release_notes option.

Android

For publishing to the Google Play Store, it seems like changelogs are required to exist in a folder structure on disk before upload_to_play_store is executed.

These differences make it difficult to understand how to properly do release notes, not to mention making a uniform release process for different release targets and environments. It would be nice if set_changelog could provide changelog for TestFlight, App Store, and Google Play Store, for instance. No matter how, some harmonization would be appreciated at least.