Backport of EE change in `app/views/search/_form.html.haml`

What does this MR do?

It integrates the chance made in EE for the file app/views/search/_form.html.haml (Merge Request in EE) to conditionally render a sub-template with render_if_exists.

This is part of the solution for the EE-issue #11040

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.