Revert "Ran tests in netcore 3 locally; have to reset to 2.2 for appveyor"; cant build either need appveyor/ci#3098

This reverts commit 9b5c3d0.

Author: ChrisMcKee

src/BCrypt.Net.UnitTests/BCrypt.Net.UnitTests.csproj

@@ -3,7 +3,7 @@
   <PropertyGroup>
     <AssemblyName>BCrypt.Net.UnitTests</AssemblyName>
     <PackageId>BCrypt.Net.UnitTests</PackageId>
-    <TargetFramework>netcoreapp2.2</TargetFramework>
+    <TargetFramework>netcoreapp3.0</TargetFramework>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|AnyCPU'">