[BuildCheck Suggestion]: Non-SDK project defines TargetFramework

[rainersigwald]

Summary

A buildcheck to catch errors like the one that is the root cause of #10629.

Background and Motivation

In the modern .NET SDK, you define <TargetFramework> or <TargetFrameworks> to control what the project output targets. Older non-SDK projects have other properties for this (like TargetFrameworkVersion). Currently it is allowed and ignored to specify TargetFramework . . . but that can lead to a lot of confusion if someone does it, thinks they have retargeted the project, and then moves on.

Sample issue or antipattern that the check should be flagging

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="15.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
  <PropertyGroup>
    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
    <ProjectGuid>22dfcebd-2972-484f-9b0c-d1c6f28626eb</ProjectGuid>
    <OutputType>Library</OutputType>
    <AppDesignerFolder>Properties</AppDesignerFolder>
    <RootNamespace>ClassLibrary2</RootNamespace>
    <AssemblyName>ClassLibrary2</AssemblyName>
    <TargetFrameworkVersion>v4.6</TargetFrameworkVersion>
    <FileAlignment>512</FileAlignment>
    <Deterministic>true</Deterministic>
	  <TargetFrameworks>net46;net8.0</TargetFrameworks> <!-- THIS IS BOGUS -->
  </PropertyGroup>

Sample output

No response