automated pr to maintain dependencies for github actions

dependabot will create PR once new versions of GH actions become available, so no need for manual updates anymore ;-)

see dependabot docs at https://docs.github.com/en/code-security/getting-started/dependabot-quickstart-guide

See also: #1820

.github/dependabot.yml

@@ -0,0 +1,7 @@
+version: 2
+updates:
+  # Maintain dependencies for GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"