Deprecate and remove Enable exact code search setting

Background

We have a setting called Enable exact code search in User Preference. It allows users to toggle between Zoekt based exact code search and Elasticsearch based code search. As exact code search matures and we are close to finish rollout on GitLab.com, we want to stop supporting Elasticsearch backend. Please note, the exact code search is compatible with the existing code search APIs and provides better UX and search quality.

Proposal

For GitLab.com, we will remove the setting. For self-managed and Dedicated instances, we may continue supporting the setting until GA.