Remove webhooks_gitlab_instance_header feature flag

What does this MR do and why?

This MR removes the webhooks_gitlab_instance_header feature flag to release the feature.

• #31333 (closed)
• #375001 (closed)

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 #375001 (closed)