Enable `performance_roadmap` feature flag by default

What does this MR do and why?

We have enabled :performance_roadmap for gitlab-org group on GitLab.com for a while and now it is time to default it to true for everyone.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

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 #337198 (closed), #299814