Kubeadm: Bump minimum supported version to v1.13 at HEAD

[luxas]

xref: https://github.com/kubernetes/kubeadm/issues/1260
this should be simpler than it is but meanwhile...

let's:

• bump the minimum version to v1.13 at HEAD
• remove v1.12-specific code
• remove v1.10, v1.11, v1.12 records for matching etcd versions

@yagonobre or @ereslibre can you help with this?

[luxas]

cc @rosti @fabriziopandini @timothysc @neolit123

[yagonobre]

I will work on it
/lifecycle active
/assign

[neolit123]

ideally https://github.com/kubernetes/kubeadm/issues/1260
should probably be done before this.

constants need cleanup first....

[luxas]

Can do it after as well, or at the same time :wink: whatever works for me as long as we do both

[fabriziopandini]

@yagonobre I guess this is done for v1.13 or I'm wrong?

[yagonobre]

I guess that we need to bump https://github.com/kubernetes/kubernetes/blob/df77b729269ca58720219b3eaddf78a5a80fbd7e/cmd/kubeadm/app/constants/constants.go#L380