GraphQL: Mutation to remove issue from board

We need a GraphQL mutation to remove an issue from a board.

Currently, this button simply removes the corresponding label or assignee or milestone. So we should do #36307 (closed), #208782 (closed) and #36312 (closed) first.

If we include those in the updateIssue mutation this may be done already, otherwise it will be a case of moving those into this mutation also.