Fix epic assignment in the createIssue mutation

What does this MR do?

This fixes #298840 (closed), in which epic assignment in the createIssue mutation was found to be broken.

There were a couple of reasons here (incorrect use of global IDs for one), but the proximate cause was the change in the name of the method in the FOSS implementation of this mutation, which broke the EE override. An overrides annotation is added to make sure this does not happen again.

Example GraphQL queries

mutation createIssue(
  $projectPath: ID!
  $epicId: EpicID
  $title: String!
  $description: String
  $labels: [String!]
) {
  createIssue(
    input: {
      projectPath: $projectPath
      epicId: $epicId
      title: $title
      description: $description
      labels: $labels
    }
  ) {
    issue {
      webUrl
    }
  }
}

with variables like:

{
  "description": "Update alert's markup in app/views/admin/application_settings/integrations.html.haml",
  "epicId": "gid://gitlab/Epic/5213",
  "labels": ["frontend", "Pajamas", "component::alert", "pajamas:integrate"],
  "projectPath": "gitlab-org/gitlab",
  "title": "GlAlert > Update markup in app/views/admin/application_settings/integrations.html.haml"
}

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

This is a bug fix with new tests.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Related to #298840 (closed)