chore: Add initial k3s test matrix

What does this MR do?

Adds a test matrix for the k3s-based tests that enables testing on multiple k8s versions.

This does raise the minimum Kubernetes version we test against to 1.16 to simplify the matrix job script, since we do not have k3s tagged with kubernetes versions below 1.16. We still support older versions, but this seems like the right tradeoff to make.

Actually, we can support 1.14-1.16. I updated the k3s test images accordingly.

Screenshots

Manual QA

• Test this change using a Cluster management project