Reduce scope of File Encoding RFC implementation #4878

I would like to suggest a staged approach for https://github.com/PowerShell/PowerShell-RFC/blob/master/2-Draft-Accepted/RFC0020-DefaultFileEncoding.md

By removing the support for WindowsLegacy as an encoding type and removing the support for PSDefaultFileEncoding, I believe we can satisfy the bulk of our customers by standardizing our various encoding parameter types to a single type System.Text.Encoding and provide a default encoding of UTF-8 without bom. A type forwarder could be constructed to support the string UTF-8WITHBOM to be a utf-8 encoding with BOM. We can use the current code when appending to a file and encoding is not specified.

If customer demand for WindowsLegacy is strong, we can implement that at a later date. This provides a staged approach while producing the breaking change earlier to gather feedback