Show cluster agents UI only when enabled on GitLab.com [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Hides cluster agents UI if not enabled on GitLab.com

Does not affect self-managed

Related issue: #300960 (closed), https://gitlab.com/gitlab-org/configure/general/-/issues/102

Screenshots (strongly suggested)

Before

After

Self-managed (no change) :

GitLab.com, not enabled:

GitLab.com, enabled:

Does this MR meet the acceptance criteria?

Conformity

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  • [-] I have not included a changelog entry because _____.
• [-] Documentation (if required)
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• Style guides
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• 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.