ci: add dependabot config for GitHub Actions

This makes dependabot open PRs to bump actions used by CI workflows,
so they don't get stale.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Author: kolyshkin

.github/dependabot.yml

@@ -0,0 +1,10 @@
+# Please see the documentation for all configuration options:
+# https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
+
+version: 2
+updates:
+  # Dependencies listed in .github/workflows/*.yml
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"