Fix kube version string to use '+' as suffix

[jnummelin]

Signed-off-by: Jussi Nummelin jnummelin@mirantis.com

Issue
In semver - signifies a pre-release, thus the reported Kubernetes server version is slightly wrong:

kubectl version
Client Version: version.Info{Major:"1", Minor:"20", GitVersion:"v1.20.2", GitCommit:"faecb196815e248d3ecfb03c680a4507229c2a56", GitTreeState:"clean", BuildDate:"2021-01-13T13:28:09Z", GoVersion:"go1.15.5", Compiler:"gc", Platform:"darwin/amd64"}
Server Version: version.Info{Major:"1", Minor:"20+", GitVersion:"v1.20.5-k0s1", GitCommit:"6b1d87acf3c8253c123756b9e61dac642678305f", GitTreeState:"clean", BuildDate:"2021-04-08T09:00:44Z", GoVersion:"go1.15.11", Compiler:"gc", Platform:"linux/amd64"}

What this PR Includes
This PR changes the k0s build kube version to use a "static" +k0s suffix as that's more correct from semver point of view.