ci: add Dependabot for GitHub Actions version updates (#69)

Keeps pinned SHA digests up-to-date automatically.

Author: lhoupert

.github/dependabot.yml

@@ -0,0 +1,17 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    cooldown:
+      default-days: 7
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    groups:
+      minor-and-patch:
+        applies-to: version-updates
+        patterns:
+          - "*"
+        update-types:
+          - "minor"
+          - "patch"