<yvals_core.h>: Update _MSVC_STL_UPDATE to June 2020

[StephanTLavavej]

Explanation on our wiki: Macro _MSVC_STL_UPDATE

Code:

STL/stl/inc/yvals_core.h

Line 488 in c64e60b

#define _MSVC_STL_UPDATE 202005L

Previous issue: #777

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