Enable requeue_on_nack for rabbitmq task queue backend

[svix-jplatte]

Motivation

I asked about this flag when porting things to omniqueue but failed to actually follow up on the feedback. Apparently true is the intended default, though this is not documented anywhere.

Solution

Use the intended default.

[svix-jplatte]

Fix for CI: #1303

[svix-jplatte]

Not merging yet because I'm not entirely certain about my understanding of intended default, should have made this a draft until it's confirmed I guess.