<yvals_core.h>: Update _MSVC_STL_UPDATE to October 2020

[StephanTLavavej]

Explanation on our wiki: Macro _MSVC_STL_UPDATE

Code:

STL/stl/inc/yvals_core.h

Line 504 in 530bdc5

#define _MSVC_STL_UPDATE 202009L

Previous issue: #1257

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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[praveenojha33]

Hello @StephanTLavavej , I have made a PR for this please review 😄 .