Mention remote name instead of "its developer"

>Equally, it might not be entirely right to say the app is "provided solely by its developer". Packages on Flathub are ideally maintained by the developers of the software, but not always.
>
>_Originally posted by @ids1024 in https://github.com/elementary/sideload/issues/80#issuecomment-742583444_

We currently say apps are provided by their developers, but that's not precise; technically all we know is that an app is provided by the distributor/remote.

See also: https://github.com/elementary/appcenter/issues/1132#issuecomment-550815517