Reduce Dependabot frequency to monthly (#623)

To reduce the churn + email notification noise slightly. GitHub will
still open PRs for any security issues outside of the monthly cadence,
plus updates can be manually triggered through the GitHub UI.

This should hopefully be one of the last of our team's repositories that
isn't already on a monthly cadence.

.github/dependabot.yml

@@ -3,15 +3,15 @@ updates:
   - package-ecosystem: "cargo"
     directory: "/"
     schedule:
-      interval: "daily"
+      interval: "monthly"
     labels:
       - "dependencies"
       - "rust"
       - "skip changelog"
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "daily"
+      interval: "monthly"
     labels:
       - "dependencies"
       - "github actions"