The ability to create a channel is not yet implemented
for the initial launch of the Flutter mobile app, so we remove
the legacy React Native app instructions.
This feature is tracked in:
https://github.com/zulip/zulip-flutter/issues/1572
Part of #34748.
Updates the content in `help/include/create-a-channel-instructions`
and `help/include/create-channels-intro` for the stream to channel
rename. Done in a separate commit from the previous so that the
rename of these files is tracked as such in the history.
