Enable advanced search by default for global search scope

What does this MR do?

Issue #197231

This MR turns on Advanced Search features (backed by Elasticsearch) for Global Search scope even if index limiting is turned on for projects/namespaces.

Documentation updates are included to call out feature behavior changes. The changes include an option to allow users to revert a search back to basic search by modifying the URL.

Note: See the issue for background on the why

Screenshots

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