ci: Add dependabot configuration (#3265)

If this works the way I think it does, we should get automatic PRs proposing github actions version updates.

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions" # See documentation for possible values
+    directory: "/" # Location of package manifests
+    schedule:
+      interval: "weekly"