Win8 baseline: Drop support for Win7 / Server 2008 R2

[StephanTLavavej]

🗺️ Overview

• This is a clean revert of Temporarily restore support for Win7 / Server 2008 R2 #4857.
• I will need to add several MSVC-internal changes when mirroring.
• Works towards Support for Win7 / Server 2008 R2 through Win8.1 / Server 2012 R2 is DOOMED #4858.
• This will not be backported.
  • For the rest of its lifecycle, VS 2022 17.14 will support Win7.

📜 Changelog

• The STL no longer supports targeting Windows 7 and Server 2008 R2:
  • Improved the performance of system_clock::now() and atomic::wait() by directly calling APIs that were added in Windows 8.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.