Reenable show_gitlab_agent_feedback feature flag by default

What does this MR do and why?

This MR reenables show_gitlab_agent_feedback by default as it was disabled in %17.0 to give space for a temporary gitops warning.

MR acceptance checklist

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Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. Visit Operate -> Kubernetes clusters page
2. Create an agent following the instructions 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. Verify the banner shown in the agent list.

Related to #440176 (closed)