chore: group github action updates into one pr

If there are more than one update to GitHub Actions dependencies
(like `actions/checkout`) at the same time, then the updates will be
grouped into a single pull request.

If the pull request remains open, then subsequent updates will be added
to it.

This should help keep the number of open pull requests down.

Author: mroderick

.github/dependabot.yml

@@ -24,3 +24,7 @@ updates:
   directory: "/"
   schedule:
     interval: "weekly"
+  groups:
+    actions-deps:
+      patterns:
+      - "*"