Default UTF-8 encoding should not include BOM

[nblumhardt]

https://github.com/serilog/serilog-sinks-file/blob/dev/src/Serilog.Sinks.File/Sinks/File/FileSink.cs#L59

The built-in Encoding.UTF8 object is problematic because it emits the byte order mark not otherwise required or expected in UTF-8 encoded files.

This should instead be a:

static readonly Encoding DefaultUtf8 = new UTF8Encoding(false);