Don't override allowed_scopes for non-ES searches

What does this MR do?

This MR is the result of an investigation into errors observed on Gitlab.com (#217158 (closed)).

When elasticsearch_search and elasticsearch_limit_indexing are both enabled, globally searching for an ES-specific scope (like commits) results in a 500 error. This is because EE::Search::GlobalService overrides allowed_scopes by only checking elasticsearch_search.

This MR changes EE::Search::GlobalService so that it checks search_using_elasticsearch?(via use_elasticsearch?) before overriding allowed_scopes. search_using_elasticsearch? provides an accurate answer even when elasticsearch_limit_indexing is on.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team