Multi-target the Models project (#124)

This will make it possible to use the json-stat2 model in PCAxis.Serializers

Co-authored-by: Rune Johansen <6797636+runejo@users.noreply.github.com>

Author: runejo

src/PxWeb.Api2.Server.Models/PxWeb.Api2.Server.Models.csproj

@@ -3,7 +3,8 @@
     <Description>Server library for PxWeb API 2.0</Description>
     <Copyright>No Copyright</Copyright>
     <Authors>SCB &amp; SSB</Authors>
-    <TargetFramework>net8.0</TargetFramework>
+    <TargetFrameworks>net8.0;netstandard2.1;netstandard2.0</TargetFrameworks>
+    <LangVersion>8.0</LangVersion>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <PreserveCompilationContext>true</PreserveCompilationContext>
     <Version>1.0.0</Version>
@@ -14,9 +15,13 @@
     <IncludeSourceRevisionInInformationalVersion>false</IncludeSourceRevisionInInformationalVersion>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="Microsoft.Extensions.Configuration.Json" Version="8.0.0"/>
+    <PackageReference Include="Microsoft.Extensions.Configuration.Json" Version="8.0.0" />
     <PackageReference Include="Microsoft.VisualStudio.Azure.Containers.Tools.Targets" Version="1.10.8" />
-    <PackageReference Include="Swashbuckle.AspNetCore.Newtonsoft" Version="6.4.0"/>
+    <PackageReference Include="Nullable" Version="1.3.1">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+    <PackageReference Include="Swashbuckle.AspNetCore.Newtonsoft" Version="6.4.0" />
     <PackageReference Include="Swashbuckle.AspNetCore.Annotations" Version="6.4.0" />
     <PackageReference Include="JsonSubTypes" Version="1.8.0" />
   </ItemGroup>