Add an alert for the users who has deprecated `gitops` keyword in their agent's configuration

Proposal

As we are removing the deprecated gitops keyword, we need to add an alert for the users who have this keyword in their agent's configuration.

Implementation plan

1. On the Kubernetes clusters page, request each local agent configuration file via the /api/:version/projects/:id/repository/files/:path/raw path.

2. Parse the file using parse util from yaml. Detect if the high-level gitops keyword is present.

3. If the keyword is found, add a non-dismissible alert to the Kubernetes clusters page.

4. Alert text (to be reviewed by the Technical Writing team:

   The pull-based deployment features of the GitLab agent for Kubernetes is deprecatedIf you use the agent for pull-based deployments, you should migrate to Flux.

5. Link to https://gdk.test:3443/help/user/clusters/agent/gitops/migrate_to_flux.md

6. Design: