Use new query for issue board users autocomplete

What does this MR do and why?

Use the new user autocomplete GraphQL query so that it includes subgroup and descendant project members

This is similar to !129348 (merged) but for boards

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #417757 (closed)