Add cluster domain warning

What does this MR do?

Adds a cluster domain warning to the Auto DevOps settings screen if no base domain is defined for a cluster.

Issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/7974

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Code review guidelines
• Style guides

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers