Set <LangVersion> to preview (#35121)

See #35100

Author: roji

Directory.Build.props

@@ -39,7 +39,7 @@
     <PackageTags>Entity Framework Core;entity-framework-core;EF;Data;O/RM;EntityFramework;EntityFrameworkCore;EFCore</PackageTags>
     <Product>Microsoft Entity Framework Core</Product>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
-    <LangVersion>12.0</LangVersion>
+    <LangVersion>preview</LangVersion>
     <Nullable>enable</Nullable>
     <DebugType>portable</DebugType>
     <PackageProjectUrl>https://docs.microsoft.com/ef/core/</PackageProjectUrl>