use only maven central for dependabot

Signed-off-by: Olivier Lamy <olamy@apache.org>

.github/dependabot.yml

@@ -21,7 +21,9 @@ registries:
   maven-central:
     type: maven-repository
     url: https://repo.maven.apache.org/maven2
-
+    username: ""
+    password: ""
+    
 updates:
   - package-ecosystem: "maven"
     directory: "/"