Enable `kubernetes_namespace_for_environment` FF by default

What does this MR do and why?

This MR enables kubernetes_namespace_for_environment FF by default. The FF is enabled on production for a week now, so it should be safe to enable it by default.

Screenshots or screen recordings

How to set up and validate locally

1. Enable KAS on your GDK (steps 1-2 from the guide).

2. Visit the Project -> Infrastructure -> Kubernetes clusters page and create an agent following the instruction from the modal.

   • Select the "Connect a cluster" button
   • The modal should pop up
   • In the modal select "Select an agent or enter a name to create new"
   • You probably won't have any configured agents to show up in the list, create a new one by typing the name of your choice
   • The button should appear at the bottom of the list saying "Create agent: <your-agent-name>"
   • Select the button and click "Register" in the next view.
   • Save the token to use it in the next point.

3. Please follow points 3-8 from the guide and then the (Optional) Deploy the GitLab Agent (agentk) with k3d section to create a local cluster and connect your agent with the cluster.

4. Add the following configuration file inside your project on the default branch: .gitlab/agents/<your-agent-name>/config.yaml:

   gitops:
     manifest_projects:
     - id: <your-group>/<your-project>
   user_access:
     access_as: 
       agent: {}
     projects:
     - id: <your-group>/<your-project>

5. Visit the Environments page:

   • Project -> Operate -> Environments if the new navigation is enabled
   • Project -> Deployments -> Environments if the new navigation is not enabled

6. Create/Edit an environment using the UI, select the agent created in the previous steps

7. Verify the behavior of the Kubernetes namespace field

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 #417129 (closed) and !125191 (merged)