[RUN AS-IF-FOSS] Move feature to core: "Multiple Kubernetes clusters"

What does this MR do?

Move ability to use multiple Kubernetes clusters to core.

Screenshots

Multiple project clusters: image

Multiple group clusters: image

Does this MR meet the acceptance criteria?

Conformity

Availability and Testing

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

Closes #212229 (closed)

Edited by Bob Van Landuyt