<yvals_core.h>: Update _MSVC_STL_UPDATE to March 2021

[StephanTLavavej]

Explanation on our wiki: Macro _MSVC_STL_UPDATE

Code:

STL/stl/inc/yvals_core.h

Line 518 in 30ac440

#define _MSVC_STL_UPDATE 202102L

Previous issue: #1618

This issue is intended for a new contributor (especially one new to GitHub) to get started with the simplest possible change.

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