Replace (broken) action-updater workflow with Dependabot (#267)

In hazelcast/hz-docs#1024 an action was added to
check GitHub Actions dependencies for updated versions, and raise PRs to
fix if identified.

This action [has been
failing](https://github.com/hazelcast/hz-docs/actions/workflows/action-updater.yml)
for 6 months due to a token issue.

[Based on the PRs it _previously_
created](hazelcast/hz-docs#1024), it's looks to
have used a former users' token which has since lost permissions.

Although [`github-actions-version-updater`] offers some additional
functionality over Dependabot (e.g. updating via SHA), for _our_
configuration this isn't required. As such we can use Dependabot instead
and require no such token configuration.

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"