Change Encoding.Default to UTF-8 *without BOM* for Unix interoperability

This is a follow-up from here.

Encoding.Default in the full framework on Windows reports the legacy "ANSI" code page (encoding).
Core doesn't support these legacy pages (by default) so its Encoding.Default must return an encoding that makes sense in a cross-platform world.
Currently, Encoding.UTF8 is returned, which is a UTF-8 encoding with BOM.
- The reason for returning Encoding.UTF8 as Encoding.Default is a historic one, it seems, as the source-code comment mentions Silverlight(!).
UTF-8 with BOM is problematic on all Unix platforms, whose utilities do not expect a BOM and instead treat it as data, leading to unexpected results.
Having Encoding.Default return a UTF-8 encoding with BOM is pointless and confusing, because it neither represents any platform's nor the framework's own true default, and may cause disruption in practice.
- Note that both .NET Framework's and .NET Core's true default is BOM-less UTF-8, because that's what's used in the absence of specifying an encoding with the methods of the System.IO class.

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