[cicd] configure dependabot (#356)

Author: FrancoisCarouge

.github/dependabot.yml

@@ -1,11 +1,7 @@
-# To get started with Dependabot version updates, you'll need to specify which
-# package ecosystems to update and where the package manifests are located.
-# Please see the documentation for all configuration options:
-# https://docs.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
-
 version: 2
 updates:
-  - package-ecosystem: "" # See documentation for possible values
-    directory: "/" # Location of package manifests
+  - package-ecosystem: "github-actions"
+    directory: "/"
     schedule:
       interval: "weekly"
+    open-pull-requests-limit: 3