GraphQL - normalise issue complexity multiplier

Similar to #205312 (closed)... but for board list issues 🙂

Since issue boards are quite complex, it's possible for clients sending GraphQL queries to hit the query complexity limit unintentionally.

While we are still figuring out board issue pagination, and ensuring we don't hit query complexity limit for legitimate queries, the complexity multiplier in board_list_issues_resolver has been set to 0.005 (half of the usual 0.01). We can modify this after we finalise the queries being used for issue boards.