Messages about overriding content and/or version URLs on STDOUT spoil JSON output

[bwarden]

If we override the content or version URLs using -u, -c, or -v, we print out a message on STDOUT warning about it: https://github.com/clearlinux/swupd-client/blob/ec4a272115476db8c07a650ef009548d801bcad9/src/swupd_lib/globals.c#L553-L572

In most cases, this is probably fine. But if we've been given -j for JSON output, we really should only be outputting valid JSON strings. We should probably move these (and any other) warnings to STDERR.

This will likely affect test cases, too.