🔧 chore: batch dependency updates weekly on Tuesday

Dependabot now checks once a week on Tuesday, with a catch-all group per
ecosystem collecting that week's bumps into a single pull request. The other
projects I maintain use the same schedule, so their update reviews arrive
together.

Author: gaborbernat

.github/dependabot.yaml

@@ -3,6 +3,11 @@ updates:
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "daily"
+      interval: "weekly"
+      day: "tuesday"
+    groups:
+      github-actions:
+        patterns:
+          - "*"
     cooldown:
       default-days: 7