Expose namespace_per_environment flag for Kubernetes clusters

What does this MR do?

Exposes the previously hidden namespace_per_environment option for new and existing clusters. Originally, this option was introduced as a legacy cluster variation, but we are re-exposing it due to user demand.

This allows users to opt-out of namespace-per-environment in favour of namespace-per-project for deployments.

See #38054 (closed)

Screenshots / manual QA

Add existing cluster

• ✅ The option is present, checked by default
• ✅ The option is written to the DB

Edit cluster

• ✅ The option is present with the current DB value
• ✅ The change is written to the DB

Add new EKS cluster

• ✅ The option is present, checked by default
• ✅ The option is written to the DB

Add new GKE clusters

• ✅ The option is present, checked by default
• ✅ The option is written to the DB

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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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