Do not use underscore in image names

When suffixing image_version (mkosi.version) do not use an underscore
since that breaks systemd-machined/machinectl because undescore are
invalid in machine names.

This is related to commit 7eb4ea8

Closes: systemd#2367

mkosi/config.py

@@ -588,7 +588,7 @@ def config_default_output(namespace: argparse.Namespace) -> str:
  output = namespace.image_id or namespace.image or "image"
 
  if namespace.image_version:
- output += f"_{namespace.image_version}"
+ output += f"-{namespace.image_version}"
 
  return output