Set search_index_integrity FF to default enabled

What does this MR do and why?

Related to #214601 (closed)

FF issue #392981 (closed)

Feature flag has been fully enabled on GitLab.com for a month without performance issues.

Recently found issue was fixed in !127246 (merged)

We plan to leave default enabled for one milestone and fully release the feature in %16.4

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

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.