The support of Leap 15.4 has ended, not much point to test build there
so move it to 15.6 (upper bound). The lower bound is still 15.3 to catch
potential backward compatibility problems.
The hub image exists (https://hub.docker.com/r/kdave/ci-opensuse-leap-15.6-x86_64).
Signed-off-by: David Sterba <dsterba@suse.com>
