Update GH actions with Dependabot (#902)

All actions are pinned with hases as suggested by OpenSSF Scorecard, as
soon as #901 lands. Actions can be upgraded with Dependabot, see
https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot.

.github/dependabot.yml

@@ -0,0 +1,12 @@
+version: 2
+updates:
+
+  # Check for updates to GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    groups:
+      github-actions:
+        patterns:
+          - "*"