Deprecate Stalebot in favor of actions/stale #14

re: probot/stale#430

Let's update our default repo configuration to use the Stale Github Action instead of maintaining a separate stale.yml file. Once we have an acceptable pattern defined here, we can apply it to the major OpenClarity projects (eg. VMClarity, KubeClarity).

This may also supersede #13 and part of openclarity/kubeclarity#432.