Cleanup Global Search terminology in App

Problem

The Advanced Global Search feature (powered by Elasticsearch) has a mismatch of naming across the application. Sometimes the feature is referred to as Advanced Global Search and sometimes the feature is referred to as Elasticsearch.

Proposal

As part of gitlab-com/www-gitlab-com!42162 (merged) the Global Search Group features will be renamed to:

Basic Global Search - represents the global search when performed against the PostgreSQL backend

Advanced Global Search - represents the global search when performed against the Elasticsearch backend

This terminology needs to be updated in both the GitLab application and the Documentation:

Docs: https://docs.gitlab.com/ee/user/search/advanced_global_search.html

Application Changes:

Current	Update