ci: Mergeable must have ready-to-merge and not require version (#14600)

you can see our init discuss in https://lists.apache.org/thread/xgcd3wndfst88ornvdknd64nol76k89n

.github/mergeable.yml

@@ -58,5 +58,5 @@ mergeable:
               regex: 'feature|bug|improvement|document|chore'
               message: 'Label must include one of the following: `feature`, `bug`, `improvement`, `document`, `chore`'
           - must_include:
-              regex: '\d+\.\d+\.\w+'
-              message: 'Label must include one or more version numbers.'
+              regex: 'ready-to-merge'
+              message: 'Please check if there are PRs that already have a `ready-to-merge` label and can be merged, if exists please merge them first.'