Warn when netstandard dll is referenced for unsupported TFMs

[Evangelink]

          Idk where that ended up, but the idea is like what system.io.hashing is doing:

<Project InitialTargets="NETStandardCompatError_System_IO_Hashing_net8_0">
  <Target Name="NETStandardCompatError_System_IO_Hashing_net8_0"
          Condition="'$(SuppressTfmSupportBuildWarnings)' == ''">
    <Warning Text="System.IO.Hashing 9.0.2 doesn't support $(TargetFramework) and has not been tested with it. Consider upgrading your TargetFramework to net8.0 or later. You may also set &lt;SuppressTfmSupportBuildWarnings&gt;true&lt;/SuppressTfmSupportBuildWarnings&gt; in the project file to ignore this warning and attempt to run in this unsupported configuration at your own risk." />
  </Target>
</Project>

Originally posted by @nohwnd in #5054 (comment)