Added .NET 7.0 library output support; also, update .NETCore3.1 test / tools to .NET 6.0 for new LTS version

Author: ritchiecarroll

src/Gemstone.PQDIF/Gemstone.PQDIF.csproj

@@ -3,7 +3,7 @@
   <Import Sdk="Microsoft.NET.Sdk" Project="Sdk.props" />
 
   <PropertyGroup>
-    <TargetFrameworks>netstandard2.0;netstandard2.1</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;netstandard2.1;net7.0</TargetFrameworks>
     <RootNamespace>Gemstone.PQDIF</RootNamespace>
     <PackageId>Gemstone.PQDIF</PackageId>
     <Authors>Grid Protection Alliance</Authors>

src/UnitTests/Gemstone.PQDIF.UnitTests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <TargetFramework>net6.0</TargetFramework>
     <IsPackable>false</IsPackable>
     <Configurations>Debug;Release;Development</Configurations>
   </PropertyGroup>