CI: adding dependabot for actions update (#14523)

@Carreau - this is what's needed for the actions update, the rest in
settings I think are doing security related things.

.github/dependabot.yml

@@ -0,0 +1,15
+# 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: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    groups:
+      actions:
+        patterns:
+          - "*"