Add a Dependabot configuration

... to keep the GitHub Actions up to date.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

.github/dependabot.yml

@@ -0,0 +1,7 @@
+version: 2
+updates:
+# https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot#enabling-dependabot-version-updates-for-actions
+  - package-ecosystem: "github-actions" # See documentation for possible values
+    directory: "/" # Location of package manifests
+    schedule:
+      interval: "weekly"