Dependabot target the master branch

Since git plugin 4.0 has released, the stable-3.x branch can be left
to sit more quietly.

Author: MarkEWaite

.dependabot/config.yml

@@ -4,7 +4,7 @@ update_configs:
   - package_manager: "java:maven"
     directory: "/"
     update_schedule: "weekly"
-    target_branch: "stable-3.x"
+    target_branch: "master"
     default_reviewers:
       - "MarkEWaite"
     default_labels: