Add `discussions` and `notes` fields for `VulnerabilityType`

What does this MR do?

Implemented Types::Notes::Noteable interface to introduce discussions and notes fields to Types::VulnerabilityType.

Removed the usage of definition_methods as it makes it really hard to extend the functionality of Gitlab CE by prepending the module defined under EE namespace. This shouldn't cause any problems as we don't have any child interface and the documentation of this method says;

Methods defined in this block will be: - Added as class methods to this interface - Added as class methods to all child interfaces

Related to #223814 (closed)

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
• [-] 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