Can't create backport when assignee is invalid

[Yu-Jack]

Not sure why it can't create backport issue in this harvester/harvester#4179.

The error message is 422 {"message": "Validation Failed", "errors": [{"value": "******", "resource": "Issue", "field": "assignees", "code": "invalid"}], "documentation_url": "https://docs.github.com/rest/issues/issues#create-an-issue"}

@bk201 : Guess that assignee is not the harvester member.

[Yu-Jack]

About assignees field:

Logins for Users to assign to this issue. NOTE: Only users with push access can set assignees for new issues. Assignees are silently dropped otherwise.