build: add required fields to dependabot configuration (#20786)

Add the missing schedule and open-pull-request-limits fields to the
dependabot configuration.

Author: josephperrott

.github/dependabot.yml

@@ -3,10 +3,13 @@ version: 2
 updates:
   - package-ecosystem: "npm"
     directory: "/"
+    schedule:
+      interval: "daily"
     commit-message:
       prefix: "build"
     labels:
       - "area: build"
       - "merge ready"
       - "merge safe"
       - "target: patch"
+    open-pull-requests-limit: 5