add support to build net472 on Linux

Author: David Coe

csharp/Directory.Packages.props

@@ -37,6 +37,7 @@
     <PackageVersion Include="Google.Cloud.BigQuery.Storage.V1" Version="3.17.0" />
     <PackageVersion Include="Google.Cloud.BigQuery.V2" Version="3.11.0" />
     <PackageVersion Include="Microsoft.IO.RecyclableMemoryStream" Version="3.0.1" />
+    <PackageVersion Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.3" />
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.4.0" />
     <PackageVersion Include="Microsoft.SourceLink.GitHub" Version="10.0.201" />
     <PackageVersion Include="Moq" Version="4.20.72" />

csharp/test/Apache.Arrow.Adbc.Tests/Apache.Arrow.Adbc.Testing.csproj

@@ -1,15 +1,15 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks Condition="'$(IsWindows)'=='true'">net8.0;net472</TargetFrameworks>
-    <TargetFrameworks Condition="'$(TargetFrameworks)'==''">net8.0</TargetFrameworks>
+    <TargetFrameworks>net8.0;net472</TargetFrameworks>
     <IsPackable>true</IsPackable>
     <IsTestProject>true</IsTestProject>
     <ProcessArchitecture>$([System.Runtime.InteropServices.RuntimeInformation]::ProcessArchitecture.ToString().ToLowerInvariant())</ProcessArchitecture>
   </PropertyGroup>
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" />
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" PrivateAssets="all" />
     <PackageReference Include="Moq" />
     <PackageReference Include="OpenTelemetry" />
     <PackageReference Include="System.Diagnostics.DiagnosticSource" />