Improve Http1ContentLengthMessageBody's reset logic

[halter73]

Addresses #19476

This prevents error logs like the following if the client closes an HTTP/1.x Content-Length request mid-body, and the last application read from the body examined data that it didn't consume:

fail: Microsoft.AspNetCore.Server.Kestrel[13]
      Connection id "0HLU8P7S8VQMH", Request id "0HLU8P7S8VQMH:00000001": An unhandled exception was thrown by the application.
System.InvalidOperationException: The examined position cannot be less than the previously examined position.
   at System.IO.Pipelines.ThrowHelper.ThrowInvalidOperationException_InvalidExaminedPosition()
   at System.IO.Pipelines.Pipe.AdvanceReader(BufferSegment consumedSegment, Int32 consumedIndex, BufferSegment examinedSegment, Int32 examinedIndex)
   at System.IO.Pipelines.Pipe.DefaultPipeReader.AdvanceTo(SequencePosition consumed)
   at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.Http1ContentLengthMessageBody.<ReadAsyncInternal>g__ResetReadingState|9_0()
   at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.Http1ContentLengthMessageBody.ReadAsyncInternal(CancellationToken cancellationToken)
   at Microsoft.AspNetCore.WebUtilities.FormPipeReader.ReadFormAsync(CancellationToken cancellationToken)

[halter73]

@Pilchie for rc2

[Pilchie]

Approved for .NET 5 RC2

[halter73]

@dotnet/aspnet-build Can someone please merge this? Thanks!