chore(ci): update Dependabot schedule (#67)

Set a schedule for Bundler and GitHub Actions from weekly to monthly.

Author: nekrich

.github/dependabot.yml

@@ -1,15 +1,10 @@
-# To get started with Dependabot version updates, you'll need to specify which
-# package ecosystems to update and where the package manifests are located.
-# Please see the documentation for all configuration options:
-# https://docs.github.com/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
-
 version: 2
 updates:
   - package-ecosystem: "bundler"
     directory: "/"
     schedule:
-      interval: "weekly"
+      interval: "monthly"
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "weekly"
+      interval: "monthly"