fix: prefer custom backoff strategy to built-in if provided

[lbennett-stacki]

This feels a bit easier to reason over to me, my thinking is...

1. A queue can define a built-in backoff strategy for use as an always-available default a. Queues no longer define their own custom strategies. Related to https://github.com/taskforcesh/bullmq/pull/2275 and https://github.com/taskforcesh/bullmq/pull/2276
2. A worker can define its own backoff strategy on that/any queue and the custom strategy will be preferred

Maybe a future piece could be to unify the backoff settings for both queues and workers. Or deciding one place to define it.