Rename search_scope in class proxy code to search_level

Background

The options param named search_scope actually contains the search level (project, group, global). Search scope is typically referring to the document type being searched (example: blobs, merge_requests, projects, etc).

Proposal

Rename the param to search_level where it is set and used.

Implementation plan

1. rename where it is set in the base_options method in:

• ee/lib/gitlab/elastic/search_results.rb
• ee/lib/gitlab/elastic/group_search_results.rb
• ee/lib/gitlab/elastic/project_search_results.rb

2. update all class proxy code where it is used:

• ee/lib/elastic/latest/epic_class_proxy.rb
• ee/lib/elastic/latest/git_class_proxy.rb
• ee/lib/elastic/latest/milestone_class_proxy.rb
• ee/lib/elastic/latest/note_class_proxy.rb
• ee/lib/elastic/latest/wiki_class_proxy.rb
• ee/lib/search/elastic/filters.rb (in dev)

3. update all specs where it is tested:

• ee/spec/lib/elastic/latest/epic_class_proxy_spec.rb
• ee/spec/lib/elastic/latest/wiki_class_proxy_spec.rb
• ee/spec/lib/search/elastic/filters_spec.rb (in dev)