Migrate available_agents_dropdown to GlCollapsibleListbox

What does this MR do and why?

This MR migrates the GlDropdown used to select or create a GitLab agent for Kubernetes to the GlCollapsibleListbox UI element.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Visit Project -> Infrastructure -> Kubernetes clusters
2. Select Connect cluster
3. Verify the agent selection/creation dropdown in the modal

To validate using GitPod you'll need to enable KAS:

1. Go to the gitlab-development-kit folder

2. Change the gdk.yml file to add the following lines:

   gitlab_k8s_agent:
     enabled: true

3. Install Bazel using Bazelisk

   npm install -g @bazel/bazelisk

4. Run gdk reconfigure and gdk restart

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
• frontend developer review
• frontend maintainer review

Related to #384287 (closed)