Standardize $(LangVersion) in netstandard2.0 projects (#1322)

The default `$(LangVersion)` for `netstandard2.0` projects is
[C# 7.3][0], which does not contain many of the niceties we would
like to use.  Throughout our history we have updated individual
projects to use the latest language features, but this has led to
inconsistencies.

Instead, standardize this in `Directory.Build.targets` to use a
common C# language version for `netstandard2.0` projects.

Note that we standardized on [C# 13.0][1] -- the default in .NET 9 --
instead of `latest`, as `latest` is discouraged because it can change
based on what versions are installed on a machine.

[0]: https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-version-history#c-version-73
[1]: https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-version-history#c-version-13

Author: jpobst

Directory.Build.targets

@@ -10,6 +10,11 @@
     <DefineConstants>$(DefineConstants);INTERNAL_NULLABLE_ATTRIBUTES</DefineConstants>
   </PropertyGroup>
 
+  <!-- Automatically support the latest C# for netstandard2.0 projects -->
+  <PropertyGroup Condition=" '$(TargetFramework)' == 'netstandard2.0' ">
+    <LangVersion>13.0</LangVersion>
+  </PropertyGroup>
+
   <!-- Add Roslyn analyzers NuGet to all projects -->
   <ItemGroup Condition=" '$(DisableRoslynAnalyzers)' != 'True' ">
     <PackageReference Include="Microsoft.CodeAnalysis.FxCopAnalyzers" Version="3.3.0">

build-tools/Java.Interop.BootstrapTasks/Java.Interop.BootstrapTasks.csproj

@@ -3,7 +3,6 @@
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
     <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
-    <LangVersion>11.0</LangVersion>
   </PropertyGroup>
 
   <Import Project="..\..\TargetFrameworkDependentValues.props" />

src/Java.Interop.Localization/Java.Interop.Localization.csproj

@@ -2,7 +2,6 @@
 
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
-    <LangVersion>8.0</LangVersion>
     <Nullable>enable</Nullable>
     <NeutralLanguage>en</NeutralLanguage>
     <SignAssembly>true</SignAssembly>

src/Java.Interop.Tools.Cecil/Java.Interop.Tools.Cecil.csproj

@@ -2,7 +2,6 @@
 
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
-    <LangVersion>8.0</LangVersion>
     <Nullable>enable</Nullable>
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>..\..\product.snk</AssemblyOriginatorKeyFile>

src/Java.Interop.Tools.Diagnostics/Java.Interop.Tools.Diagnostics.csproj

@@ -2,7 +2,6 @@
 
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
-    <LangVersion>8.0</LangVersion>
     <Nullable>enable</Nullable>
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>..\..\product.snk</AssemblyOriginatorKeyFile>

src/Java.Interop.Tools.JavaCallableWrappers/Java.Interop.Tools.JavaCallableWrappers.csproj

@@ -2,7 +2,6 @@
 
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
-    <LangVersion>11.0</LangVersion>
     <Nullable>enable</Nullable>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <SignAssembly>true</SignAssembly>

src/Java.Interop.Tools.JavaSource/Java.Interop.Tools.JavaSource.csproj

@@ -2,7 +2,6 @@
 
   <PropertyGroup>
     <TargetFramework>$(DotNetTargetFramework)</TargetFramework>
-    <LangVersion>8.0</LangVersion>
     <Nullable>enable</Nullable>
   </PropertyGroup>
 

src/Java.Interop.Tools.Maven/Java.Interop.Tools.Maven.csproj

@@ -2,8 +2,6 @@
 
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
-    <LangVersion>11.0</LangVersion>
-    <Nullable>enable</Nullable>
     <Nullable>enable</Nullable>
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>..\..\product.snk</AssemblyOriginatorKeyFile>

src/Java.Interop/Java.Interop.csproj

@@ -27,8 +27,6 @@
     <OutputPath>$(ToolOutputFullPath)</OutputPath>
     <DocumentationFile>$(ToolOutputFullPath)Java.Interop.xml</DocumentationFile>
     <JNIEnvGenPath>$(BuildToolOutputFullPath)</JNIEnvGenPath>
-    <LangVersion Condition=" '$(JIBuildingForNetCoreApp)' == 'True' ">9.0</LangVersion>
-    <LangVersion Condition=" '$(LangVersion)' == '' ">8.0</LangVersion>
     <Version>$(JICoreLibVersion)</Version>
     <Standalone Condition=" '$(Standalone)' == '' ">true</Standalone>
   </PropertyGroup>