[SPARK-56600][K8S] Promote SparkPod to Stable

[dongjoon-hyun]

What changes were proposed in this pull request?

This PR aims to promote the SparkPod case class to @Stable from Apache Spark 4.2.0.

Why are the changes needed?

• The case class SparkPod has been served without any modification since Spark 2.4.0 and a new method was added at 3.0.0.
  • [SPARK-22839] [K8s] Refactor to unify driver and executor pod builder APIs #20910
  • [SPARK-25815][k8s] Support kerberos in client mode, keytab-based token renewal. #22911
• Since Apache Spark 3.2.0 promoted it as a developer API, it has been serving until now stably.
  • [SPARK-33261][K8S] Add a developer API for custom feature steps #30206
• We had better promote it to Stable from Apache Spark 4.2.0.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Pass the CIs.

Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Opus 4.7

[dongjoon-hyun]

Could you review this PR, @peter-toth ?

[dongjoon-hyun]

Thank you always, @peter-toth !

[dongjoon-hyun]

Merged to master for Apache Spark 4.2.0.