Fix the TargetFramework for Blazor WASM server app (#14865)

Author: mkArtakMSFT

src/Components/Blazor/Templates/src/content/BlazorWasm-CSharp/Server/BlazorWasm-CSharp.Server.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk.Web">
 
   <PropertyGroup>
-    <TargetFramework>$(DefaultNetCoreTargetFramework)</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
     <LangVersion>7.3</LangVersion>
   </PropertyGroup>