Use Ubuntu 20.04 as the default image for Kubernetes 1.18+

[hakman]

As discussed in Kops Office Hours from Jun 5, 2020.

Fixes #9163

[johngmyers]

/lgtm

[rdrgmnzs]

Thanks @hakman !

/lgtm
/approve

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[mosheavni]

If my Kubernetes version is v1.15.12, is it problem to use the Ubuntu 20.04 image?

[hakman]

@MosheM123 there should be no problem to use Ubuntu 20.04 with older versions of k8s, but you will have to use a newer version of kOps that supports it and set the k8s version manually to 1.15.x.

[mosheavni]

@MosheM123 there should be no problem to use Ubuntu 20.04 with older versions of k8s, but you will have to use a newer version of kOps that supports it and set the k8s version manually to 1.15.x.

I'm using kOps latest (1.18.2) with kubernetes 1.15.12.
Thanks for confirming @hakman !!!