Calm dependabot down a little

We added dependabot in #105 (Oct 2023) and it got very excited proposing
daily updates to github.com/aws/aws-sdk-go. Eventually it gave up.

Maybe monthly is enough to stop dependencies falling behind, without
quite so much noise?

.github/dependabot.yml

@@ -3,4 +3,9 @@ updates:
 - package-ecosystem: gomod
   directory: /
   schedule:
-    interval: daily
+    interval: monthly
+  reviewers:
+    - "buildkite/@agent-stewards"
+  ignore:
+    - dependency-name: "*"
+      update-types: ["version-update:semver-patch"]