Upgrade ServiceStack.Memory to net6.0

Author: mythz

src/ServiceStack.Memory/ServiceStack.Memory.csproj

@@ -1,10 +1,13 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>netcoreapp2.1</TargetFrameworks>
+    <TargetFrameworks>netcoreapp2.1;net6.0</TargetFrameworks>
   </PropertyGroup>
   <PropertyGroup Condition=" '$(TargetFramework)' == 'netcoreapp2.1' ">
     <DefineConstants>$(DefineConstants);NETSTANDARD;NETCORE;NETCORE2_1</DefineConstants>
   </PropertyGroup>
+  <PropertyGroup Condition=" '$(TargetFramework)' == 'net6.0' ">
+    <DefineConstants>$(DefineConstants);NETSTANDARD;NETCORE;NET6_0</DefineConstants>
+  </PropertyGroup>
   <ItemGroup>
     <ProjectReference Include="..\ServiceStack.Text\ServiceStack.Text.csproj" />
   </ItemGroup>