Add disable_vulnerability_feedback_controller feature flag

What does this MR do and why?

Introduces a disable_vulnerability_feedback_controller feature flag that tells Projects::VulnerabilityFeedbackController to return an empty response for the #index method.

Calls to -/vulnerability_feedback are the main culprit for the error budget consumption in groupthreat insights insights.

While the long-term solution is being delivered as part of Deprecate Vulnerabilities::Feedback (&5629 - closed), in the meantime we want to be able to enable/disable the response so alleviate performance concerns.

Enabling this feature flag will cause a degradation in the security MR widget. When enabled, dismissed findings do not show a Dismissed badge, and the user must click on the finding to see its state.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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 #388066 (closed)