Migrate GitLab Runner autoscaling from Docker Machine to the fleeting plugin architecture

Steps

1. Understand the timeline: the Docker Machine executor was deprecated in GitLab 17.5 and is scheduled for removal in GitLab 20.0 (May 2027); migrate before that date
2. Install a fleeting plugin for your cloud provider (e.g., fleeting-plugin-aws, fleeting-plugin-googlecompute, or fleeting-plugin-azure) alongside your existing GitLab Runner binary
3. In config.toml, replace the [runners.machine] section with an [runners.autoscaler] block that references the plugin binary path and a [runners.autoscaler.plugin_config] block for provider-specific settings such as instance type and image ID
4. Set capacity_per_instance, max_instances, and idle_time under [runners.autoscaler] to control how aggressively the runner scales up and how long idle instances wait before termination
5. Choose executor: docker+autoscaler for Docker-based jobs (equivalent to the old docker+machine) or instance for jobs that run directly on the provisioned VM without Docker
6. Run gitlab-runner verify and check the runner's log output to confirm instances are being created and jobs dispatched before decommissioning the old Docker Machine configuration