Add GraphQL mutation to unlink security policy project from the project

Why are we doing this work

We have already introduced a way to assign security policy project do selected project, however we also need a way to unlink this project to disable security policies for selected projects. This issue addresses that with new GraphQL mutation.

Relevant links

• Epic
• Design Issue

Non-functional requirements

• Documentation: update GraphQL documentation,
• [-] Feature flag:
• [-] Performance:
• Testing: verify if you can link the project, then unlink security policy project using GraphQL and then again link the same project to make sure linking/unlinking is reversible

Implementation plan

• backend Add new service to ee/app/services/security/orchestration/unassign_service.rb that will remove association between Policy project and selected project,
• backend Add mutation SecurityPolicyProjectUnassign to ee/app/graphql/mutations/security_policy/unassign_security_policy_project.rb, make sure only that this is authorized for users with permission to update_security_orchestration_policy_project, this new mutation should accept one argument projectPath (GraphQL::Types::ID) and will call UnassignService internally