Run workhorse in feature specs

What does this MR do?

Currently we go rspec -> capybara -> puma -> rails application code

We want to go rspec -> capybara -> workhorse -> puma -> rails application code

This will allow us to test various things that rely on workhorse better than we can at present. It will also mean that gitlab-rails pipelines will fail if the contents of GITLAB_WORKHORSE_VERSION aren't a valid Workhorse reference.

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

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

Closes #199431 (closed)