ci: Dependabot monthly instead of daily (cost) (#958)

Co-authored-by: bee-san <[email protected]>

.github/dependabot.yml

@@ -3,15 +3,15 @@ updates:
 - package-ecosystem: "github-actions"
   directory: "/"
   schedule:
-    interval: "daily"
+    interval: "monthly"
 - package-ecosystem: "docker"
   directory: "/"
   schedule:
-    interval: "daily"
+    interval: "monthly"
 - package-ecosystem: cargo
   directory: "/"
   schedule:
-    interval: daily
+    interval: monthly
   open-pull-requests-limit: 10
   ignore:
   - dependency-name: anyhow