Change GitHub actions dependabot schedule to weekly (#19981)

Main motivator for this is the `taiki-e` action which has lots of
version bumps and subsequently lots of PRs; but also the other actions
are weekly only so might as well make them consistent.

Author: Jefffrey

.github/dependabot.yml

@@ -48,7 +48,7 @@ updates:
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "daily"
+      interval: "weekly"
     open-pull-requests-limit: 10
     labels: [auto-dependencies]
   - package-ecosystem: "pip"