Add clusters association to deployment

What does this MR do?

Persist cluster for a Deployment.

A deployment should really persist the cluster instead repeatedly querying deployment_platform after the fact (in the MR widget). Also, re-querying the deployment platform might choose the wrong platform as clusters get added/deleted/edited. See https://gitlab.com/gitlab-org/gitlab-ce/issues/63475. We will drop the fallback to #deployment_platform_cluster in a later MR

So we persist the cluster we get from deployment_platform when we create the deployment, and use this persisted cluster from then on.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team