Rename the Agent on the GitLab UI

What does this MR do and why?

As we are renaming the GitLab Kubernetes Agent to GitLab Agent for Kubernetes we are updating the related UI with the revised copies.

New copies are:

1. In the Agents' empty state view on Infrastructure -> Kubernetes page:

Use the [GitLab Agent](https://gitlab.com/help/user/clusters/agent/index.md) to safely connect your Kubernetes clusters to GitLab.
You can deploy your applications, run your pipelines, use Review Apps, and much more.

2. In the Clusters' empty state view on Infrastructure -> Kubernetes page:

Connect your cluster to GitLab through [cluster certificates](https://gitlab.com/help/user/infrastructure/clusters/index.md#certificate-based-kubernetes-integration-deprecated).

If you are setting up multiple clusters with Auto DevOps, learn
[how to use multiple Kubernetes clusters](https://gitlab.com/help/user/project/clusters/multiple_kubernetes_clusters) first.
The certificate-based method to connect clusters to GitLab was [deprecated](https://about.gitlab.com/blog/2021/11/1

3. In the Agents' modal empty state variant:

Install a new agent

Before installing an agent, add the agent's configuration file to this repository.

[What's the Agent's configuration file?](https://gitlab.com/help/user/clusters/agent/install/index)
[Go to the repository](LINK-TO-REPO)
Before using the GitLab Agent, ask a GitLab Administrator to enable the
[GitLab Agent Server (KAS)](https://gitlab.com/help/administration/clusters/kas).

4. In the Agents' modal for Agent configuration present variant:

Select an agent to install

Select an agent to register with GitLab and install on your cluster. 

[How to register an agent?](https://gitlab.com/help/user/clusters/agent/install/index.md)

5. In the alert on Security & Compliance -> Policies -> Create new policy page: To enable alerts, [install an agent](https://gitlab.com/help/user/clusters/agent/install/index.md) first.

Updated screenshots

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. To validate the Agents' and Clusters' empty states:
   • Go to Infrastructure -> Kubernetes clusters
   • Validate the page
2. To validate the Agent's registration modal in the empty state:
   • Select Actions button
   • enable/disable the KAS to validate the related text
3. To validate the Agent's connect modal with the configurations present:
   • add the agent configurations as per the docs
4. To validate the alert to add Agent:
   • Go to Security & Compliance -> Policies -> Create new policy page

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.

Related to #346197 (closed)