ci: enable dependabot for github actions

Weekly check for newer commit SHAs / tags of every action used in
.github/workflows/. Updates are batched into a single PR via the
'actions' group, with ci-prefixed commit messages to match existing
conventions.

Author: ShortArrow

.github/dependabot.yml

@@ -0,0 +1,12 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    commit-message:
+      prefix: "ci"
+    groups:
+      actions:
+        patterns:
+          - "*"