Update Github Actions with Dependabot

This repo currently uses actions which use a deprecated Node.js
version. Dependabot will make sure that we use up-to-date actions.

Signed-off-by: Antonin Bas <antonin.bas@broadcom.com>

.github/dependabot.yml

@@ -0,0 +1,8 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    # Workflow files stored in the default location of `.github/workflows`
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 5