Kubernetes UI updates

What does this MR do and why?

This MR removes duplicative buttons on the /clusters page and lowercases the agent for consistency. As part of the MR I've also made some clean-up.

The changes:

1. On the All tab, remove the Install a new agent button, per this issue/comment.

2. On the All tab, remove the Connect existing cluster button

3. On the Agent tab, remove the Connect with the GitLab Agent button (it's a duplicate of the button on the top right and likely not controlled by the feature flag)

4. On the Certificate tab, remove the Connect existing cluster button

5. Clean-up of the GlEmptyState for the agent's and certificate's empty states. Together with the clean-up I removed the related CSS and changed the description slot to title. Using the description slot led to creating an empty h1 tag, while the title slot produces more clean and meaningful HTML.

6. Lowercase all instances of the Agent string:

   • on the empty state
   • in the Recommended tooltip:

   

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Visit Infrastructure -> Kubernetes clusters page.
2. View the different tabs.

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
• UI text review
• QA review
• test Quality review
• frontend developer review
• frontend maintainer review

Related to #357028 (closed) and #355682 (closed)