Resolve "Validate k8s credentials provided at cluster creation"

What does this MR do?

Displays error messages on the Cluster creation screen if K8s credentials are invalid.

EE port: gitlab-ee!12226

Closes #55447 (closed)

Preview

status === 'unreachable' (cluster_status.json)

status === 'authentication_failure' (cluster_status.json)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance 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