Expose missing disable mention value on group API

What does this MR do?

This MR is a follow-up of !20184 (merged) and exposes the mentions_disabled Group setting (introduced in GitLab 12.6) via the group API (/api/v4/groups/:id).

Does this MR meet the acceptance criteria?

Question

• While the GraphQL API for project exposes almost all project attributes, the group one (app/graphql/types/group_type.rb) is pretty much empty. What is the goal with the group GraphQL API? Shall I also expose the mentions_disabled there?

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