mirror: Enable Dependabot updates for GitHub Actions

Let Dependabot create Pull Requests to enable the GitHub Actions in use
in the repository, to avoid breakage such as the one recently
experienced with the actions/upload-artifact Action for the release
workflow.

Signed-off-by: Quentin Monnet <qmo@kernel.org>

Author: qmonnet

.github/dependabot.yml

@@ -0,0 +1,12 @@
+# https://docs.github.com/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
+
+version: 2
+
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    target-branch: "main"
+    commit-message:
+      prefix: "ci:"