[SPARK-35405][DOC] Submitting Applications documentation has outdated information about K8s client mode support
[SPARK-35405][DOC] Submitting Applications documentation has outdated information about K8s client mode support

### What changes were proposed in this pull request?
[Submitting Applications doc](https://spark.apache.org/docs/latest/submitting-applications.html#master-urls) has outdated information about K8s client mode support.
It still says "Client mode is currently unsupported and will be supported in future releases".
![image](https://user-images.githubusercontent.com/31073930/118268920-b5b51580-b4c6-11eb-8eed-975be8d37964.png)

Whereas it's already supported and [Running Spark on Kubernetes doc](https://spark.apache.org/docs/latest/running-on-kubernetes.html#client-mode) says that it's supported started from 2.4.0 and has all needed information.
![image](https://user-images.githubusercontent.com/31073930/118268947-bd74ba00-b4c6-11eb-98d5-37961327642f.png)

Changes:
![image](https://user-images.githubusercontent.com/31073930/118269179-12b0cb80-b4c7-11eb-8a37-d9d301bbda53.png)

JIRA: https://issues.apache.org/jira/browse/SPARK-35405

### Why are the changes needed?
Outdated information in the doc is misleading

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

### How was this patch tested?
Documentation changes

Closes #32551 from o-shevchenko/SPARK-35405.

Authored-by: Oleksandr Shevchenko <d7261134785940bf1cfc6aaf2704c7ac81437448@datarobot.com>
Signed-off-by: Dongjoon Hyun <9b02cb7e37158797a8e22c559f7c5f5ecd28ea57@apple.com>

HyukjinKwon/spark,chuckchen/spark,milliman/spark,zero323/spark,ueshin/apache-spark,hvanhovell/spark,holdenk/spark,xuanyuanking/spark,WeichenXu123/spark,wangyum/spark,dongjoon-hyun/spark,witgo/spark,maropu/spark,chuckchen/spark,wangmiao1981/spark,rednaxelafx/apache-spark,cloud-fan/spark,nchammas/spark,rednaxelafx/apache-spark,rednaxelafx/apache-spark,wangmiao1981/spark,xuanyuanking/spark,taroplus/spark,chuckchen/spark,zzcclp/spark,gengliangwang/spark,srowen/spark,ueshin/apache-spark,hvanhovell/spark,srowen/spark,wangyum/spark,milliman/spark,zzcclp/spark,vinodkc/spark,wangyum/spark,zero323/spark,JoshRosen/spark,chuckchen/spark,zero323/spark,milliman/spark,JoshRosen/spark,taroplus/spark,chuckchen/spark,srowen/spark,rednaxelafx/apache-spark,holdenk/spark,milliman/spark,HyukjinKwon/spark,hvanhovell/spark,zzcclp/spark,maropu/spark,jiangxb1987/spark,cloud-fan/spark,HyukjinKwon/spark,zero323/spark,hvanhovell/spark,zero323/spark,dongjoon-hyun/spark,HyukjinKwon/spark,apache/spark,xuanyuanking/spark,shaneknapp/spark,shaneknapp/spark,mahak/spark,nchammas/spark,nchammas/spark,HyukjinKwon/spark,cloud-fan/spark,xuanyuanking/spark,zzcclp/spark,maropu/spark,rednaxelafx/apache-spark,taroplus/spark,shaneknapp/spark,JoshRosen/spark,wangyum/spark,mahak/spark,xuanyuanking/spark,cloud-fan/spark,gengliangwang/spark,vinodkc/spark,dongjoon-hyun/spark,rednaxelafx/apache-spark,WeichenXu123/spark,dongjoon-hyun/spark,cloud-fan/spark,holdenk/spark,shaneknapp/spark,WeichenXu123/spark,shaneknapp/spark,wangmiao1981/spark,ueshin/apache-spark,gengliangwang/spark,vinodkc/spark,gengliangwang/spark,apache/spark,HyukjinKwon/spark,HyukjinKwon/spark,dongjoon-hyun/spark,witgo/spark,xuanyuanking/spark,xuanyuanking/spark,chuckchen/spark,ueshin/apache-spark,vinodkc/spark,ueshin/apache-spark,taroplus/spark,wangyum/spark,apache/spark,WeichenXu123/spark,dongjoon-hyun/spark,gengliangwang/spark,vinodkc/spark,witgo/spark,srowen/spark,holdenk/spark,witgo/spark,mahak/spark,jiangxb1987/spark,nchammas/spark,witgo/spark,apache/spark,vinodkc/spark,holdenk/spark,cloud-fan/spark,zzcclp/spark,JoshRosen/spark,JoshRosen/spark,maropu/spark,wangmiao1981/spark,milliman/spark,jiangxb1987/spark,wangmiao1981/spark,wangyum/spark,gengliangwang/spark,jiangxb1987/spark,mahak/spark,zzcclp/spark,shaneknapp/spark,ueshin/apache-spark,WeichenXu123/spark,wangmiao1981/spark,cloud-fan/spark,holdenk/spark,gengliangwang/spark,jiangxb1987/spark,jiangxb1987/spark,vinodkc/spark,maropu/spark,hvanhovell/spark,nchammas/spark,taroplus/spark,mahak/spark,witgo/spark,nchammas/spark,milliman/spark,srowen/spark,maropu/spark,dongjoon-hyun/spark,WeichenXu123/spark,witgo/spark,nchammas/spark,mahak/spark,apache/spark,shaneknapp/spark,mahak/spark,chuckchen/spark,apache/spark,zero323/spark,taroplus/spark,milliman/spark,srowen/spark,JoshRosen/spark,holdenk/spark,hvanhovell/spark,rednaxelafx/apache-spark,zero323/spark,srowen/spark,wangmiao1981/spark,WeichenXu123/spark,apache/spark,JoshRosen/spark,jiangxb1987/spark,zzcclp/spark,wangyum/spark,taroplus/spark,maropu/spark,hvanhovell/spark,ueshin/apache-spark