ci: reduce frequency of dependabot nags

.github/dependabot.yml

@@ -3,11 +3,11 @@ updates:
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: weekly
+      interval: monthly
     open-pull-requests-limit: 10
 
   - package-ecosystem: "gomod"
     directory: "/"
     schedule:
-      interval: weekly
+      interval: monthly
     open-pull-requests-limit: 10