Port netfx configuration changes to master

[ericstj]

For packages that still build in 5.0 bring forward servcing changes from 3.x that added netfx configurations.

• Microsoft.XmlSerializer.Generator
• System.Collections.Immutable
• System.Composition.AttributedModel
• System.Composition.Convention
• System.Composition.Hosting
• System.Composition.Runtime
• System.Composition.TypedParts
• System.Diagnostics.DiagnosticSource
• System.IO.Pipelines
• System.Net.Http.Json
• System.Net.Http.WinHttpHandler
• System.Net.WebSockets.WebSocketProtocol
• System.Numerics.Tensors
• System.Reflection.Metadata
• System.Reflection.MetadataLoadContext
• System.Resources.Extensions
• System.Text.Encoding.CodePages
• System.Text.Encodings.Web
• System.Threading.Channels
• System.Threading.Tasks.Dataflow

[joperezr]

I thought I had an issue for this already, thanks for logging @ericstj I'll take care of it.