packagegroup-core-ssh-dropbear: Add openssh-sftp-server recommendation

packagegroup-core-ssh-dropbear: Add openssh-sftp-server recommendation

Seems sad to have to do this but openssh is moving to use sftp instead
of scp to move files. This means scp from Fedora 36 will no longer be
able to move files to/from a dropbear based image. This breaks a number
of our key QA tests and I suspect will cause users pain too.

The sftp server from openssh is small (200kb uncompressed) and standalone
so adding it to the packagegroup seems to be the best way to preserve user
sanity. If people really don't want it, they can just use dropbear instead
of the packageground.

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