Use FreeBSD autoscaler for "stress" tests

The FreeBSD autoscaler has been configured to utilize the new "instance"
GitLab Runner executor to spawn "stress" test CI jobs on AWS EC2
dynamically. A shared GitLab Runner named "freebsd-instance-autoscaler"
has been set up in GitLab CI/CD to communicate with EC2, provisioning VM
instances on demand based on a FreeBSD 13 AMI image. This image is the
same as the one previously used for FreeBSD "stress" tests before the
implementation of autoscaling (specifically, the
"freebsd13-amd64-bind9stress.aws.lab.isc.org" GitLab Runner in CI/CD).

(cherry picked from commit 65a93b77e1)

.gitlab-ci.yml

@@ -99,14 +99,16 @@ stages:
     - runner-manager
     - aarch64
 
-# Stress-testing runners
+# Autoscaling GitLab Runner on AWS EC2 (FreeBSD)
 
 .freebsd-stress-amd64: &freebsd_stress_amd64
   tags:
-    - amd64
+    - bsd-stress-test
     - aws
-    - bsd-stress
-    - stress
+    - autoscaler
+    - shell
+    - stress-test
+    - amd64
 
 ### Docker Image Templates