Update info about preview gallery ( poshtestgallery.com )

[o-l-a-v]

Summary of the new feature / enhancement

According to this issue poshtestgallery.com was taken down/private two+ years ago:

• PowerShell/PowerShell#17019

But it works now: https://www.poshtestgallery.com/

---

According to this changelog:

• https://github.com/PowerShell/PowerShellGallery/blob/master/PreviewReleaseNotes.md

The test/preview gallery URL is https://www.preview.powershellgallery.com/, but it does not work.

Proposed technical implementation details (optional)

• Update info on what URL is the official, actual preview/test PowerShell Gallery.
• Update info on what's the purpose of this test gallery is.
• A changelog would be nice. :) Both for the preview gallery and the production one.