bump version to v28.0.0-dev
This file is only used as default if no version is specified. We
should probably get rid of this, but let's update it to better
reflect the version that developer builds are building.
https://github.com/docker/cli/blob/d48fb9f9f7bdb6e0ef37dbde68612a1704cad46e/docker.Makefile#L22
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Sebastiaan van Stijn