Fix OpenStruct use in queue_duration_logger

What does this MR do and why?

Fixes OpenStruct use in spec/lib/gitlab/grape_logging/loggers/queue_duration_logger_spec.rb since Ruby 3 discourages it.

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 #344279 (closed)