[SPARK-38007][K8S][DOCS] Update K8s doc to recommend K8s 1.20+
[SPARK-38007][K8S][DOCS] Update K8s doc to recommend K8s 1.20+

### What changes were proposed in this pull request?

This PR aims to update K8s `Prerequisites` doc to recommend K8s 1.20+ and to remove `Spark 2.3`.

### Why are the changes needed?

The AS-IS document is outdated because it's written 4 years ago.
- kubernetes-client 4.1.1 dropped K8s 1.6 and 1.7 support. (https://github.com/fabric8io/kubernetes-client)
- kubernetes-client 5.8.0 dropped K8s 1.9 support. (https://github.com/fabric8io/kubernetes-client)
- EKS also makes the following EOLs (https://docs.aws.amazon.com/eks/latest/userguide/kubernetes-versions.html)
  - 1.16 (September 27, 2021)
  - 1.17 (November 2, 2021)
  - 1.18 (March 31, 2022)
  - 1.19 (April, 2022)
- MKS has the similar status (https://docs.microsoft.com/en-us/azure/aks/supported-kubernetes-versions?tabs=azure-cli#aks-kubernetes-release-calendar)

### Does this PR introduce _any_ user-facing change?

No. This is a documentation change.

### How was this patch tested?

Manual.

Closes #35306 from dongjoon-hyun/SPARK-38007.

Authored-by: Dongjoon Hyun <6ff48b33cbb828151035af58a92f18a39f1d1036@apache.org>
Signed-off-by: Dongjoon Hyun <6ff48b33cbb828151035af58a92f18a39f1d1036@apache.org>
(cherry picked from commit e362ef17e20a5a3957b4b03949940b7787462b42)
Signed-off-by: Dongjoon Hyun <6ff48b33cbb828151035af58a92f18a39f1d1036@apache.org>

milliman/spark,milliman/spark,milliman/spark,milliman/spark,milliman/spark,milliman/spark,milliman/spark