chore: add dependabot for github-actions

Enable weekly Dependabot updates for github-actions so the SHA-pinned
action references stay current.

Author: nunomaduro

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly