FR: Firestore C++ SDK for desktop out of beta

### [REQUIRED] Please fill in the following fields:
* Main Firebase Components in concern: Firestore + Auth
* Platform you are using the C++ SDK on: Mac and Windows
* Platform you are targeting: Desktop

### [REQUIRED] Please describe the question here:
I notice on the readme that there is no reference to Firestore C++. I found some old issues from 2019 that talk about firestore being in alpha.

I'm interested in using the Firestore C++ SDK in a Windows and Mac desktop application. What is the state of firestore currently? Is desktop supported for Windows and Mac environments, or would I be treading into alpha/beta territory?

Start with the repository README and the existing Firestore and Auth documentation referenced by the issue. Determine whether Windows and Mac desktop support is documented and whether the issue needs a documentation update or only a maintainer response. Done means the supported status and beta state are clearly recorded for desktop users.

Written by the indexing model from the issue text.