Add dependabot.yml.

Author: fniephaus

.github/dependabot.yml

@@ -0,0 +1,15 @@
+version: 2
+updates:
+
+  - package-ecosystem: "maven"
+    directory: "/"
+    schedule:
+      interval: "month"
+  - package-ecosystem: "gradle"
+    directory: "/"
+    schedule:
+      interval: "month"
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "month"