Add locked as an argument to updateIssue

What does this MR do?

Follow up to: !36866 (comment 380255488)

Not blocking at all, since it's in line with the other issue mutations we have. But should we also make that field an argument of the Update mutation, just like the confidential argument?

Adds the ability to lock and unlock discussion on a issue as part of an update via GraphQL.

Example query:

mutation {
  updateIssue(input: { projectPath: "h5bp/html5-boilerplate", iid: "9", locked:true }) {
    issue {
      iid
      healthStatus
      discussionLocked
      weight
    }
    errors
  }
}

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