Allow user to revert back to non-ElasticSearch search

What does this MR do?

When there are no search results using Elasticsearch we want to give people the option to revert to Postgres search. This mitigates risks with rolling out Elasticsearch to our users in case there are data or query problems they will always be able to get back to the same old postgres experience.

Screenshots

When there are no results

After switching to basic search

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• 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

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

#33678 (closed)