bitbake/conf: Default to zstd compressed image output

bitbake/conf: Default to zstd compressed image output

Switch our default qemu images to use .zst compressed images by default
since this is the output format we release during the release process
and is the one that users would prefer to download. This makes the release
process use the actual generated output from the system and avoids post
processing.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>