Create and write to deployment_clusters table

What does this MR do?

Split out of !20983 (merged)

Creates a table "deployment_clusters" to expand the cluster-related deployment details from just cluster_id to include kubernetes namespaces as well, and adds double writes to this table. In the next release, we can then copy the remaining data over, start reading exclusively from deployment_clusters, and add an ignore for deployments.cluster_id

Storing the namespace used is important for accurate deploy boards, because Environment#deployment_namespace is the namespace for the next deployment, and this can change as cluster configuration changes.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
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• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
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Security

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• [-] 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
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