Fix ConfigurationManager framework build

Author: MihaZupan

src/libraries/System.Configuration.ConfigurationManager/src/System.Configuration.ConfigurationManager.csproj

@@ -11,6 +11,7 @@
     <!-- TODO https://github.com/dotnet/runtime/issues/90400: Annotate for nullable reference types -->
     <Nullable>disable</Nullable>
     <NoWarn>$(NoWarn);nullable</NoWarn>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <IncludeSpanPolyfills>true</IncludeSpanPolyfills>
   </PropertyGroup>