Make the ".NET Core Linux" project files compatible with version 3.1.x

dotnet/dotnet.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
+  </PropertyGroup>
+
+</Project>

dotnet/project.json

@@ -1,19 +0,0 @@
-{
-  "version": "1.0.0-*",
-  "buildOptions": {
-    "debugType": "portable",
-    "emitEntryPoint": true
-  },
-  "dependencies": {},
-  "frameworks": {
-    "netcoreapp1.0": {
-      "dependencies": {
-        "Microsoft.NETCore.App": {
-          "type": "platform",
-          "version": "1.0.1"
-        }
-      },
-      "imports": "dnxcore50"
-    }
-  }
-}