Disable clusters API if :certificate_based_clusters is disabled

What does this MR do and why?

Disables the management API endpoints for legacy, certificate-based clusters if the :certificate_based_clusters feature flag is disabled.

#351675 (closed)

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How to set up and validate locally

1. Create a personal access token
2. Test successful request:

   $ curl --header "Private-Token: <token>" http://172.16.123.1:3000/api/v4/admin/clusters
   []

3. Disable the feature flag:

   Feature.disable :certificate_based_clusters

4. Test unsuccessful request:

   $ curl --header "Private-Token: <token>" http://172.16.123.1:3000/api/v4/admin/clusters
   {"message":"404 Not Found"}

MR acceptance checklist

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