Improve automation around kubectl versions to include in images

[mallardduck]

This issue is to track the initial phase steps to add basic metadata to Rancher release branches to track supported k8s versions. The goal is to add in additional metadata to build.yaml in the r/r repo that we can then consume to know what versions to target.

---

Implementation Details

Essentially this means adding a new field to build.yaml on each active branch. For main branch or 2.11, we would expect this to look like:

kubernetesVersions: [v1.29, v1.30, v1.31, v1.32]

This field does not need to be rendered into the constants.go file that build.yaml generates into.

---

Future Consideration

Once implemented in each current release branch of Rancher, we could create a new rancher/k8s-metadata repo to aggregate these datapoints into a single location. This can improve the UX/DX of accessing this data allowing for it to be more easily used in CI/CD contexts. This could be

[mallardduck]

rancher/rancher#48601