Allow Windows 1909 for Docker executor

What does this MR do?

Allow Windows 1909 for Docker executor

Why was this MR needed?

Allow users to run docker-windows on Docker hosts that run on Windows 1909. The helper image is not provided by GitLab at the moment and will be handled in the future #6212 (closed)

Before: https://gitlab.com/steveazz/playground/-/jobs/502350018 After: https://gitlab.com/steveazz/playground/-/jobs/502347970

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

• Reference #6212 (closed)
• Helper image might be handled in #4396 (closed)