Create availability policy for reference images, so we can prune them

[mboersma]

/kind feature

Describe the solution you'd like

CAPZ documentation should make clear that the project provides reference images only for the last X patch releases of Kubernetes, for each of the last Y minor releases. Then maintainers should prune our stored images down to those covered by the availability policy.

Background

To facilitate tests, and to help new users kick the tires, the CAPZ project has published reference images for each version of Kubernetes released over the last couple years. But there are costs associated with keeping the entire set of reference images available.

The number of images involved has become quite large and poses some challenges for Azure's Marketplace. We run into issues frequently due to the size of the overall offer. We are also billed for the amount of storage used, which continues to grow.

Additionally, these images are for testing and reference. They aren't "supported" in the sense that Azure usually supports its services and binary artifacts. They are not updated with OS-level security patches, and many Kubernetes releases remain there after they have been effectively obsoleted by newer versions with CVE fixes.

The danger is that even with making the availability policy clear and taking a deliberate approach to removing old images (such as Kubernetes 1.16.10!), there may be users who depend on a particular image and are surprised. Since these images were never supported, and we have made clear that after a trial phase, users need to maintain their own images, I think we are covered. But I would love any other suggestions to help avoid this.

Anything else you would like to add: