Update instructions for Windows GCM (now mergeable)

[mavaddat]

• [x] I provide my work under the project license.
• [x] I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

Changes

Microsoft/Git-Credential-Manager-for-Windows was archived on 2020 Sep 30 and it is no longer being maintained.

The cross-platform Git Credential Manager Core (GCM Core) is the official replacement.

Therefore, I have updated the documentation to reflect this fact and recommend instead that readers rely on Git Credential Manager Core (GCM Core).

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