Fix opensearch for anonymous users

What does this MR do?

While testing !52583 (merged) on staging, I noticed that because of the :block_anonymous_global_searches feature flag we prevent loading of a new opensearch action, which results in anonymous users failure to add the GitLab search engine.

This MR excludes opensearch from before_action :block_anonymous_global_searches

Steps to repeat the issue

1. Feature.enable(:block_anonymous_global_searches) in the rails console
2. curl -q -v http://localhost:3000/search/opensearch.xml will return a redirect instead of a 200 response

Screenshots (strongly suggested)

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