Add support for Windows Server core, version 20H2 [Semi-Annual Channel release]

What does this MR do?

Adds support to the runner for running on Windows 2009.

Why was this MR needed?

Windows Server 20H2 became available on GCP:

$ gcloud compute images list --filter windows-server-20h2
NAME                                   PROJECT        FAMILY             DEPRECATED  STATUS
windows-server-20h2-dc-core-v20210511  windows-cloud  windows-20h2-core              READY

What's the best way to test this MR?

Run CI job using Windows 20H2 runner

1. Create windows 20H2 VM on GCP:

   gcloud compute instances create ppombeiro-20h2 --image https://www.googleapis.com/compute/v1/projects/windows-cloud/global/images/windows-server-20h2-dc-core-v20210511

2. Install Docker

3. RDP inside of the machine

4. Use binary published in https://gitlab-runner-downloads.s3.amazonaws.com/pedropombeiro/27220/add-windows-2009-support/index.html.

   Invoke-WebRequest -UseBasicParsing -OutFile gitlab-runner.exe https://gitlab-runner-downloads.s3.amazonaws.com/pedropombeiro/27220/add-windows-2009-support/binaries/gitlab-runner-windows-amd64.exe

5. Register runner

   .\gitlab-runner register --executor "docker-windows" --url "https://gitlab.com/" --description "docker-20H2-runner" --tag-list "docker,20H2,gitlab.com" --locked="false" --access-level="not_protected" --registration-token="$TOKEN" --non-interactive --docker-image mcr.microsoft.com/windows/servercore:20H2 --docker-helper-image pombeirp/gitlab-runner-helper:x86_64-c59b71c6-servercore20H2

6. Start runner

   .\gitlab-runner run

What are the relevant issue numbers?

Closes #27220 (closed)