Windows CI

[derrabus]

I'm opening this issue as a follow-up to #6269.

We are currently using AppVeyor for our Windows CI where we install SQLSRV extensions from PECL. However, PECL hasn't received fresh DLLs for over a year now and likely won't ever ship new DLLs. PECL does not have a build system for Windows anymore. Our workaround is to pin our build to the last release that PECL has DLLs for, but that won't hold forever.

If we drop PHP 8.1 one fine day, we must have a different solution.

• Microsoft publishes DLLs for the SQLSRV extensions on https://github.com/Microsoft/msphpsql. We could use those instead.
• Alternatively, we could evaluate a switch to GitHub actions for Windows builds as well and delegate this problem to the SetupPHP action.