Move Linux "stress" tests to autoscaled instances

The autoscaling GitLab CI runners currently used for most GitLab CI jobs spin up AWS EC2 instances that are at least as powerful as the dedicated instances used for running "stress" tests. Move all Linux-based "stress" tests to autoscaling GitLab CI runners to enable deprovisioning AWS instances reserved for running "stress" tests. Leave FreeBSD "stress" tests intact as there is currently no support for autoscaling BSD instances.

(This is nice not just because of the cost savings, but also because it enables lots of "stress" tests to run in parallel.)

Validation pipeline: https://gitlab.isc.org/isc-projects/bind9/-/pipelines/149387

See also isc-private/devops#195 & isc-private/bind-qa!85