Remove the form-based GKE / EKS creation page

What does this MR do and why?

This MR removes the form-based GKE / EKS creation page (previously available from Infrastructure -> Kubernetes clusters Actions menu. The feature of certificate-based clusters is deprecated. We are still planning to support it via a feature flag for the existent clusters, but we want to prevent users from creating new clusters with the deprecated method.

The MR removes the form-based GKE / EKS creation page, removes corresponding links from the Actions menu and related specs. It also removes the feature documentation and the cross-docs links.

Screenshots or screen recordings

The page that is removed:

The change in the Actions menu

Before	After

How to set up and validate locally

1. Visit Infrastructure -> Kubernetes clusters page
2. Verify the Create a cluster (certificate - deprecated) option is not available.

or

1. Try visiting /<namespace>/<project name>/-/clusters/new page
2. Verify getting 404.

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.

Review progress:

• UX review
• test Quality
• backend developer review
• backend maintainer review
• frontend developer review
• frontend maintainer review
• documentation review

Related to #357309 (closed)