Convert stylesheets/pages/clusters.scss into page specific bundle

What does this MR do and why?

This MR extracts page-specific CSS for the clusters and cluster_agents pages to page_bundles

Screenshots or screen recordings

No visual changes

How to set up and validate locally

1. Go to Project -> Infrastructure -> Kubernetes clusters page to verify the clusters view looks as expected
2. Create an agent within the Connect cluster button and click on it's name to verify the cluster_agents vire looks as expected

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:

• backend developer review
• backend maintainer review
• frontend developer review
• frontend maintainer review

Related to #239803 (closed)