Add GitLab agent image tag to install command

What does this MR do and why?

Currently, the commands used to install the GitLab agent (agentk) will always install the latest chart version, which, by default, uses the latest released agent. This could cause problems on older versions of GitLab, as we only guarantee N-1 version compatibility (new agents will not always work with <= N-2 GitLab versions). This makes it so that the version we install is the same as the KAS version (for every KAS version, there is a corresponding agentk version).

Screenshots or screen recordings

master	this branch

How to set up and validate locally

1. Check out this branch
2. Visit any project
3. Navigate Infrastructure > Kubernetes clusters > Connect a cluster
4. Type an arbitrary name
5. Select Create agent: ${name}
6. Press Register
7. Observe that the command includes --set image.tag=v15.0.0 or whatever is in your GITLAB_KAS_VERSION file at the GitLab root.

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.