ci: monthly dependabot for all actions (#94)

Author: lgray

.github/dependabot.yml

@@ -4,8 +4,7 @@ updates:
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "daily"
-    ignore:
-      # Official actions have moving tags like v1
-      # that are used, so they don't need updates here
-      - dependency-name: "actions/*"
+      interval: "monthly"
+    commit-message:
+      # Prefix all commit messages with "npm: "
+      prefix: "ci: "