Change Auto charset to mean UTF-8 off-Windows (#23664)

Change Auto charset to mean UTF-8 off-Windows (#23664)

Match Mono's behavior by changing the Auto character set to mean UTF-8 on non-Windows platforms (new behavior) and UCS-2/UTF-16 on Windows (current behavior).

Fixes #23464
Fixes dotnet/corefx#32442

Impact of breaking change: It is highly unlikely that anyone is actively using current behavior since it is inconsistent with Mono and doesn't match any native system APIs on non-Windows platforms (they're all UTF-8 based).

We will need to update our documentation to reflect this updated behavior.