Deploy to Kubernetes namespace specified in CI template

What does this MR do?

If an unmanaged cluster is selected for deployment and the CI template specifies a namespace (using functionality added in !20270 (merged)), use this namespace for the deployment.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability 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

#27630 (closed)