Change certificate_based_clusters FF default to false

What does this MR do and why?

This change will hide certificate-based cluster connection related
features by default globally. The FF can still be used to bring the
features back if needed. This is part of the effort of
&6940

Changelog: changed

Related issues

• FF rollout: #353410 (closed)
• 15.0 Deprecation of cert-based clusters features: gitlab-org/configure&8

Screenshots or screen recordings

The FF has been tested and set globally to false on staging since 28th April: https://gitlab.slack.com/archives/CLM200USV/p1651168460401109

How to set up and validate locally

1. Make sure that you don't have the FF toggled in anyway in your local env: Feature.remove(:certificate_based_clusters)
2. Go to any Group page of your and verify that the Kubernetes left-side menu does not show up.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.