Replace (broken) `action-updater` workflow with Dependabot

[JackPGreen]

In https://github.com/hazelcast/hz-docs/pull/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 for 6 months due to a token issue.

Based on the PRs it previously created, 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.

Post-merge actions:

• [ ] remove the ACTION_UPDATER secret

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