ci: switch to weekly dependabot updates

Apparently some dependencies get updated much more often
than I would have exepected.

It can always be triggered manually at https://github.com/systemd/systemd/network/dependencies
if there are any urgent updates

.github/dependabot.yml

@@ -5,5 +5,5 @@ updates:
   - package-ecosystem: "github-actions"
     directory: "/"
     schedule:
-      interval: "daily"
+      interval: "weekly"
     open-pull-requests-limit: 2