Enable PowerShell Core support in docker-windows executor

Description

Enable PowerShell Core 7.0.3 support in the docker-windows executor

Proposal

• Update the Windows helper image to have pwsh installed and also PowerShell 5 installed, for example !1855 (diffs)
• Update https://gitlab.com/gitlab-org/gitlab-runner/-/blob/058d0c5e98afbb54eb2d12614ed73b563109038c/docs/executors/docker.md#L52-56 to make it clear that users can use nanoserver now.
• Ensure that pwsh is called instead of powershell when necessary.

Reference

• Investigation issue #3291 (closed)
• PoC !1855 (closed)