Move the conan api concern to a proper location

🌲 Context

Follow up of !46867 (merged) for Conan packages.

In !46867 (merged), we organized Grape API concerns in lib/api/concerns. This was done to avoid having them in lib/api which should host only api classes.

🔬 What does this MR do?

The conan API is using an endpoint which is lib/api. This MR will simply move it to lib/api/concerns.

No features were added or removed. The rspecs remained unchanged.

Screenshots (strongly suggested)

n / a

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