We were inconsistent throughout the codebase and would sometimes
use the slices package and other times use our own equivalents
in api/.
This removes our versions in favor of the golang.org/x package that
does the same, which has the added benefit of reducing the surface
area of the public API module.
Note: despite existing uses of the slices package, for some reason
it didn't show up in go.mod or go.sum. Fixed that too.
## What
First part of the Kubernetes [Discovery RFD](https://github.com/gravitational/teleport/pull/13376/) to introduce a Kubernetes server per cluster.
This PR introduces a separate Kubernetes server that uses the already introduced `KubernetesClusterV3`.
## Compatibility
In previous versions, Kubernetes Clusters were part of regular `ServerV2` resource and this refactoring deprecates the `ServerV2` usage but keeps them for compatibility with previous version.
Everything is backward compatible, so v10 kubernetes agents and trusted clusters can connect fine.
## Next steps
Once this is merged, a new PR will introduce dynamic registration for Kubernetes Clusters discovered through EKS Discovery.
Zac Bergquist