Stl: <yvals_core.h>: Update _MSVC_STL_UPDATE to July 2020

[StephanTLavavej]

Explanation on our wiki: Macro _MSVC_STL_UPDATE

Code:
https://github.com/microsoft/STL/blob/5ef22f2a884706442257cea3dd9d5a6186f15c07/stl/inc/yvals_core.h#L490

Previous issue: #895

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

[morrme]

Thank you so much for making tasks like this available for newcomers!

I am new to this project but not to GitHub. I will subscribe just in case no one comes along 😄

[God-father1]

Can I work on this issue ?, I'm new to opensource and willing to contribute though I'm not new to GitHub. I would be glad to contribute.

[God-father1]

Created my first ever Opensource contribution PR. #989
Thank you! for providing the opportunity @StephanTLavavej, sir.