Fix: GraphQL breaking change `assigneeUsername` changed to `[String!]`

What does this MR do?

A small MVC to restore the previous API schema of username_assignee to a single String rather than an array and limit the incident list filter to a single assignee at a time.

See: #254705 (closed)

See also !42377 (merged) which introduced the breaking change unintentionally.

Screenshots

No visual change.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry - regression in the same release
• Documentation (if required) - auto-generated GraphQL
• 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