Add support for Windows Server core, version 2004 [Semi-Annual Channel release]

Overview

Windows Server, version 2004 (Datacenter, Standard) is the latest version released for Windows Server, this was released in 2020-05-07.

Proposal

1. Add 2004 to support list for Docker executor (!2459 (merged))
2. Create a new helper image based on 2004-amd64 and build with CI (!2459 (merged))
   1. Prerequisite: Install docker on the windows-server-2004-dc-core-v20200908 which will be used to build the helper image. (gitlab-org/ci-cd/shared-runners/images/gcp/windows-containers#30 (closed))
3. Install runner managers based on Windows 2004
   1. Register 2 new project runners under gitlab-org/gitlab-runner 👉 #26420 (comment 451079404)
   2. Add configuration for 2004 runners in https://ops.gitlab.net/gitlab-com/gl-infra/ci-infrastructure-windows 👉 https://ops.gitlab.net/gitlab-com/gl-infra/ci-infrastructure-windows/-/merge_requests/70
   3. Deploy runner managers
4. Run 2004 tests on master, similar to 1903/1909 (!2459 (merged))

Requirements

1. Use the windows-server-2004-dc-core-v20200908 image provided by GCP since we need that windows version to build the helper image. Make sure that that image has Docker installed.
2. We need GitLab Runner inside on a Windows 2004 image.