Turn on dependabot for 4.0 branch

Author: rjeberhard

.github/dependabot.yml

@@ -5,12 +5,17 @@
 
 version: 2
 updates:
-  - package-ecosystem: "maven" # See documentation for possible values
-    directory: "/" # Location of package manifests
+  - package-ecosystem: "maven"
+    directory: "/"
     schedule:
       interval: "daily"
-  - package-ecosystem: "maven" # See documentation for possible values
+  - package-ecosystem: "maven"
+    target-branch: release/4.0
+    directory: "/"
+    schedule:
+      interval: "daily"
+  - package-ecosystem: "maven"
     target-branch: release/3.4
-    directory: "/" # Location of package manifests
+    directory: "/"
     schedule:
       interval: "daily"