Update GitHub for Windows/macOS sections (it's now called GitHub Desktop)

[HonkingGoose]

Which version of the book is affected?

Problem is with source files on master.

Describe the bug:

GitHub for Windows/macOS is called GitHub Desktop now.
The landing page for the project has also changed, and you can switch between macOS and Windows downloads on the landing page.

Steps to reproduce:

1. Go to https://github.com/progit/progit2/blob/master/book/A-git-in-other-environments/sections/guis.asc
2. Click on link https://windows.github.com/
3. Click on link https://mac.github.com/
4. Note that both links redirect to GitHub Desktop landing page.
5. We also use old terms for the GitHub client, instead of using the GitHub Desktop name.
6. The images are also outdated.

Expected behavior:

Section is up to date with correct terms, and correct links.
We should also probably update the explanations to be current with the behavior/interface of GitHub Desktop.

Screenshots:

Additional context:

I went looking for GitHub Desktop references in the book, because of PR #1622.
And I noticed we should change some things.

[HonkingGoose]

@ben What are your thoughts on this issue?

GitHub for Windows and Github for macOS are now condensed into one product: "GitHub Desktop".
The links and terms should be updated whatever we do.

What do you want to do with the outdated images and short explanatory text?
Do you want to update the images and text? Or do you want to scrap the images and text?

I shall make my case for removal of the images and text:

• Images of interfaces get outdated fast
• Having text that refers to interface implementation also get outdated fast as well
• Not having images saves us the work of updating them when the interface changes
• I think how to use GitHub Desktop is explained well in their own docs, so why should we duplicate the effort.