Update LangVersion to C# 12.

Now that we have moved to net 8 from net 6, and with us no longer supporting the mono runtime, we can raise the C# version to match that supported by the net 8 runtime.

This unlocks C# 10, C# 11 and C# 12 features previously unavailable to us.
- https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-version-history#c-version-10
- https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-version-history#c-version-11
- https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-version-history#c-version-12

A newer version of StyleCop is required to avoid rules tripping up on the new syntax.

Enable a handful of style rules that don't have many violations.

Directory.Build.props

@@ -5,7 +5,7 @@
     <CopyLocalLockFileAssemblies>true</CopyLocalLockFileAssemblies>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <Optimize>true</Optimize>
-    <LangVersion>9</LangVersion>
+    <LangVersion>12</LangVersion>
     <DebugSymbols>true</DebugSymbols>
     <EngineRootPath Condition="'$(EngineRootPath)' == ''">..</EngineRootPath>
     <OutputPath>$(EngineRootPath)/bin</OutputPath>
@@ -52,7 +52,7 @@
 
   <!-- StyleCop/Roslynator -->
   <ItemGroup>
-    <PackageReference Include="StyleCop.Analyzers" Version="1.2.0-beta.435" PrivateAssets="All" />
+    <PackageReference Include="StyleCop.Analyzers" Version="1.2.0-beta.556" PrivateAssets="All" />
     <PackageReference Include="Roslynator.Analyzers" Version="4.2.0" PrivateAssets="All" />
     <PackageReference Include="Roslynator.Formatting.Analyzers" Version="4.2.0" PrivateAssets="All" />
   </ItemGroup>