Check dependencies monthly (#114)

Weekly dependency changes are more frequent than necessary for this
plugin.

Use the dependabot definition from the plugin archetype.

.github/dependabot.yml

@@ -1,10 +1,19 @@
+# Per https://docs.github.com/en/github/administering-a-repository/configuration-options-for-dependency-updates
 version: 2
 updates:
-- package-ecosystem: maven
-  directory: /
-  schedule:
-    interval: weekly
-- package-ecosystem: github-actions
-  directory: /
-  schedule:
-    interval: weekly
+
+  # Maintain dependencies for maven
+  - package-ecosystem: "maven"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    labels:
+      - "dependencies"
+
+  # Maintain dependencies for GitHub actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    labels:
+      - "skip-changelog"