Adjust maven.remoteRepositories for Artifactory cutover (#5327)

- Add `mavenCentral` remote rep
- Edit `springRepo` (`libs-snapshot` -> `/snapshot`)

See #5321

Author: onobc

spring-cloud-starter-dataflow-server/src/main/resources/dataflow-server.yml

@@ -8,8 +8,10 @@ info:
 
 maven:
   remoteRepositories:
+    mavenCentral:
+      url: https://repo.maven.apache.org/maven2
     springRepo:
-      url: https://repo.spring.io/libs-snapshot
+      url: https://repo.spring.io/snapshot
 
 spring:
   application: