Only show available services

What does this MR do?

In https://gitlab.com/gitlab-org/gitlab-ce/issues/39217 we want to remove the Kubernetes integration but not risk actually deleting customer access data. Instead we filter for available services only.

I wanted to do https://gitlab.com/gitlab-org/gitlab-ce/issues/42430 while I'm in the area but this N+1 query is harder to fix than it appears (the error comes from the first time the admin visits the admin page). The controller and views need to be reworked to not try create all services on first visit.

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