imp - Cause FTBFS when compiler versions conflict

---

To avoid obscure errors, we need to FTBFS early when we encounter CS9057.

---

Type: imp
Breaking: False
Doc Required: False
Backport Required: False
Part: 1/1

Directory.Build.props

@@ -9,7 +9,7 @@
         <LangVersion>latest</LangVersion>
         <Deterministic>true</Deterministic>
         <TargetFrameworks>net48;net8.0</TargetFrameworks>
-        <WarningsAsErrors>CS8002,nullable</WarningsAsErrors>
+        <WarningsAsErrors>CS9057,CS8002,nullable</WarningsAsErrors>
         <DebugType>portable</DebugType>
         <SignAssembly>True</SignAssembly>
         <RootPath>$(MSBuildThisFileDirectory)</RootPath>