Fix attribute inheritance in workers

What does this MR do and why?

Only feature_category was correctly inherited, in addition to sidekiq_options which comes from Sidekiq.

For the other attributes, we need to switch to get_class_attribute in order to also lookup values on parents.

Prompted by !88558 (comment 968190263).

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.