[k8s] Stabilise `k8s.pod.cpu.time` metric

[ChrsMark]

This issue tracks the effort for stabilising the k8s.pod.cpu.time metric. This is the first effort to release any of the k8s metrics as stable and hence it will be used a prototype procedure for the rest to come. Bellow the required steps are listed:

• stabilise the k8s.pod entity: [k8s] Stabilise k8s.pod entity #3002
• move the k8s.pod.cpu.time to release_candidate
• move the k8s.pod.cpu.time to stable

For all of the above steps a significant majority of the K8s SIG will need to approve the respective PRs.
SemConv maintainers will also provide the final review where significant majority is required too.

/cc @open-telemetry/semconv-k8s-approvers @jsuereth (and @open-telemetry/semconv-system-approvers since the procedure should be applicable there too)

TODOs

• verify requirement level. At the moment the metric is recommended
• TBA