Properly fix assembly loading issue, retarget BizHawk.Common again

temp fix was 9ffef2531

BizHawk.Client.EmuHawk/BizHawk.Client.EmuHawk.csproj

@@ -115,6 +115,9 @@
 		<Reference Include="SlimDX"
 			HintPath="$(SolutionDir)References/x64/SlimDX.dll"
 			Private="true" />
+		<Reference Include="System.ComponentModel.Annotations, Version=4.2.1.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"
+			HintPath="$(SolutionDir)References/System.ComponentModel.Annotations.dll"
+			Private="true" />
 	</ItemGroup>
 	<ItemGroup>
 		<Reference Include="Microsoft.CSharp" />

BizHawk.Common/BizHawk.Common.csproj

@@ -3,10 +3,10 @@
 	<Import Project="$(SolutionDir)CommonNullable.props" />
 	<PropertyGroup>
 		<AllowUnsafeBlocks>true</AllowUnsafeBlocks>
-		<TargetFramework>net48</TargetFramework>
+		<TargetFrameworks>netstandard2.0;netcoreapp3.1</TargetFrameworks>
 	</PropertyGroup>
 	<ItemGroup>
-		<PackageReference Include="System.ComponentModel.Annotations" Version="4.7.0" PrivateAssets="All" />
+		<PackageReference Include="System.ComponentModel.Annotations" Version="4.7.0" />
 		<PackageReference Include="System.Reflection.Emit.Lightweight" Version="4.7.0" PrivateAssets="All" />
 		<PackageReference Include="StyleCop.Analyzers" Version="1.2.0-beta.113"
 			Condition=" '$(MachineRunAnalyzersDuringBuild)' != '' " />

BizHawk.Emulation.Cores/BizHawk.Emulation.Cores.csproj

@@ -36,7 +36,7 @@
 	<ItemGroup>
 		<PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0" PrivateAssets="All" />
 		<!--<Reference Include="System" />-->
-		<Reference Include="System.ComponentModel.DataAnnotations" />
+		<!--<Reference Include="System.ComponentModel.DataAnnotations" />-->
 		<!--<Reference Include="System.Core" RequiredTargetFramework="3.5" />-->
 		<!--<Reference Include="System.Data" />-->
 		<!--<Reference Include="System.Data.DataSetExtensions" RequiredTargetFramework="3.5" />-->