Improve autoscaling executor providers shutdown

⚠ Built on top of !4005 (merged)! ⚠

This MR is blocked by gitlab-org/fleeting/taskscaler!24 (merged).

After taskscaler change will get merged, we need to update go.mod reference to this new version.

What does this MR do?

Improves new autoscaler's shutdown by ensuring that all managed instances are downscaled on the shutdown request (requires taskscaler update from gitlab-org/fleeting/taskscaler!24 (merged))

Why was this MR needed?

Please read gitlab-org/ci-cd/shared-runners/infrastructure#47 (closed) for justification

What's the best way to test this MR?

What are the relevant issue numbers?

Closes gitlab-org/ci-cd/shared-runners/infrastructure#47 (closed)

Closes !4005 (merged)