Runtime: Change platform code for "OSX" to "macOS" or similar

[aaronfranke]

Background and Motivation

The name "OSX" is no longer correct. The name "Mac OS X" has been deprecated since 2016 when Apple rebranded their Mac operating system to "macOS". However, in addition to this, the name "OSX" is now just flat out wrong, since the "X" stands for "10", but macOS Big Sur is going to be version 11.0.

Proposed API

Renaming "OSX" and "osx" to either "macOS" or "MacOS" or "macos" or similar would solve this problem.

This proposal does not have specific signature requirements.

Usage Examples

N/A

Alternative Designs

"macOS" (stylized), "MacOS" (PascalCase), "macos" (lowercase), and "MACOS" (uppercase) are all similar designs that would be appropriate depending on the desired naming convention.

Risks

This would break compatibility, but wrappers and aliases can be kept as needed, and upgrading shouldn't be too big of an issue.

[Dotnet-GitSync-Bot]

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[huoyaoyuan]

https://github.com/dotnet/runtime/issues/43313#issuecomment-707261960