Bump target frameworks

Suggestion made by @bording in #741

#741 (comment)

Author: lukebakken

projects/client/Apigen/Apigen.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>netcoreapp2.1</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
     <AssemblyName>Apigen</AssemblyName>
     <OutputType>Exe</OutputType>
     <PackageId>Apigen</PackageId>

projects/client/Unit/Unit.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>netcoreapp2.1</TargetFrameworks>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
     <AssemblyName>Unit</AssemblyName>
     <PackageId>Unit</PackageId>
     <GenerateRuntimeConfigurationFiles>true</GenerateRuntimeConfigurationFiles>
@@ -21,7 +21,7 @@
     <PackageReference Include="NUnit3TestAdapter" Version="3.15.1" />
     <PackageReference Include="PublicApiGenerator" Version="10.0.2" />
   </ItemGroup>
-  <PropertyGroup Condition=" '$(TargetFramework)' == 'netcoreapp2.1' ">
+  <PropertyGroup Condition=" '$(TargetFramework)' == 'netcoreapp3.1' ">
     <DefineConstants>$(DefineConstants);CORECLR</DefineConstants>
   </PropertyGroup>
 </Project>