Medium GitLab SaaS runners on Linux now available to all tiers

What this post added

Introduced medium GitLab SaaS runners on Linux for all tiers, including Free tier users. These runners utilize GCP n2d-standard-4 VMs with 4 vCPUs, 16GB RAM, and 50GB storage, and can be accessed by adding the `saas-linux-medium-amd64` tag in `gitlab-ci.yml`.