Fix k8s namespace resolution for new deployments

What does this MR do?

When working on !25845 (merged) I found a leftover use of the last successful deployment for resolving the namespace for the next deployment. This means custom namespaces are not always persisted correctly in DeploymentCluster, and hence don't always show up correctly on the job show page 😢

I already fixed several such cases in !20983 (merged), but this one slipped through the cracks 😕

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
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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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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