retain the logs for more builds (#13645)

build/Jenkinsfile.branch

@@ -18,7 +18,7 @@ pipeline {
   }
 
   options {
-    buildDiscarder(logRotator(numToKeepStr: '3'))
+    buildDiscarder(logRotator(daysToKeepStr: '15'))
     timeout(time: 2, unit: 'HOURS')
   }