Add dependabot for automated dependency updates

Dependabot will perform weekly checks of the Github actions used in CI.
When a newer version is found a pull request is opened to suggest a lift.

.github/dependabot.yml

@@ -0,0 +1,11 @@
+# Automated dependency updates.
+#
+# For configuration options see:
+# https://docs.github.com/en/github/administering-a-repository/configuration-options-for-dependency-updates
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 10