Change real-time assignees to use GraphQL subscriptions

What does this MR do?

This changes the implementation of real-time assignees from plain ActionCable to GraphQL subscriptions over ActionCable.

Before, we just sent a {event: 'updated'} payload to the client via ActionCable and then the client initiates a GraphQL API request to fetch the assignees. With GraphQL subscriptions, this is no longer needed. The client would just specify a query when they subscribe and then that query is executed every time that subscription is triggered.

Related to #244934 (closed)

QA Steps

1. Run yarn to fetch new JS depencies, then gdk restart webpack.
2. Open the issue page in two tabs, and change the assignee on one tab.

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