dependabot: monthly is plenty (dart-lang/string_scanner#54)

pkgs/string_scanner/.github/dependabot.yml

diff --git a/pkgs/string_scanner/.github/dependabot.yml b/pkgs/string_scanner/.github/dependabot.yml
index 430a85e..d5262be 100644
--- a/pkgs/string_scanner/.github/dependabot.yml
+++ b/pkgs/string_scanner/.github/dependabot.yml
@@ -8,4 +8,4 @@
   directory: "/"
   schedule:
     # Check for updates to GitHub Actions every weekday
-    interval: "daily"
+    interval: "monthly"