<yvals_core.h>: Update _MSVC_STL_UPDATE to May 2020

[StephanTLavavej]

Explanation on our wiki: Macro _MSVC_STL_UPDATE

Code:

STL/stl/inc/yvals_core.h

Line 486 in 2f4c579

#define _MSVC_STL_UPDATE 202004L

Previous issue: #670

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

[Nishikoh]

I'd like to tackle this issue.

[CaseyCarter]

I'd like to tackle this issue.

Please do!