Move VulnerabilityEntity to API entities

What does this MR do?

When the VulnerabilityEntity was introduced, it was mistakenly placed in ee/spec/serializers but this is the place for controller-related entities (as they are used by serializers).

This MR moves VulnerabilityEntity where it should have been from the beginning - to ee/lib/ee/api/entities.rb. Also, we start to properly use the API entity class (API::Entities::ProjectIdentity) to represent the embedded project attribute.

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