[MINOR][INFRA] Add enabled_merge_buttons to .asf.yaml explicitly
[MINOR][INFRA] Add enabled_merge_buttons to .asf.yaml explicitly

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

This PR aims to add the AS-IS `enabled_merge_buttons` policy explicitly. The AS-IS policy was introduced via  https://issues.apache.org/jira/browse/INFRA-18656.

### Why are the changes needed?

Currently, this policy is maintained as a self-serving manner. Here is the official documentation. It would be great if we have this explicitly for new comers.
- https://cwiki.apache.org/confluence/display/INFRA/git+-+.asf.yaml+features#Git.asf.yamlfeatures-Mergebuttons

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

No. This is a committer-only feature and there is no change in terms of the policy.

### How was this patch tested?

N/A

Closes #33505 from dongjoon-hyun/minor.

Authored-by: Dongjoon Hyun <6ff48b33cbb828151035af58a92f18a39f1d1036@apache.org>
Signed-off-by: Dongjoon Hyun <6ff48b33cbb828151035af58a92f18a39f1d1036@apache.org>

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