Retarget the nbgv tool against netcoreapp3.1 now that netcoreapp2.1 is out of service

Author: AArnott

src/nbgv/nbgv.csproj

@@ -4,7 +4,7 @@
     <ToolCommandName>nbgv</ToolCommandName>
     <PackAsTool>True</PackAsTool>
     <OutputType>Exe</OutputType>
-    <TargetFramework>netcoreapp2.1</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
     <RootNamespace>Nerdbank.GitVersioning.Tool</RootNamespace>
     <Description>A .NET Core Tool that can install, read and set version information based on git history, using Nerdbank.GitVersioning.</Description>
   </PropertyGroup>