Remove support for Windows Server 1903 image

Overview

Windows 1903 reach end of life on 08/12/2020. According to our support policy we should be able to remove support for this since it's no longer maintained by Microsoft.

Proposal

1. Support from it inside of GitLab Runner code base, just like we did in !2033 (merged)
   • Running tests
   • Publishing images
   • Docker executor version check
2. Remove infrastructure around it
   1. Remove VM image publishing which is used for CI https://gitlab.com/gitlab-org/ci-cd/shared-runners/images/gcp/windows-containers
   2. Remove autoscaler installation in https://ops.gitlab.net/gitlab-com/gl-infra/ci-infrastructure-windows/-/blob/c065a7aa04c958ffe941c0a80fbd4dba592e0c1a/ansible/group_vars/gcp_role_runner_manager.yml#L44-56
   3. Unregister the runners from the gitlab-org/gitlab-runner Settings > CI/Cd > runners section