Update to .NET Core 3.1 (#397)

Author: AlekseyMartynov

appveyor.yml

@@ -50,7 +50,7 @@ test_script:
     OpenCover\tools\OpenCover.Console -returntargetcode -register:user -oldstyle
     -target:dotnet.exe
     -targetargs:"vstest
-    net\DevExtreme.AspNet.Data.Tests.EFCore3\bin\Debug\netcoreapp3.0\DevExtreme.AspNet.Data.Tests.EFCore3.dll
+    net\DevExtreme.AspNet.Data.Tests.EFCore3\bin\Debug\netcoreapp3.1\DevExtreme.AspNet.Data.Tests.EFCore3.dll
     "
     -filter:"+[DevExtreme*]* -[DevExtreme.AspNet.Data.Tests]* -[DevExtreme.AspNet.Data.Tests.*]*"
     -output:coverage_netcore3.xml

net/DevExtreme.AspNet.Data.Tests.EFCore3/DevExtreme.AspNet.Data.Tests.EFCore3.csproj

@@ -1,12 +1,12 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netcoreapp3.0</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
     <DefineConstants>EFCORE3</DefineConstants>
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" Version="3.0.0" />
+    <PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" Version="3.1.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.7.0" />
     <PackageReference Include="xunit" Version="2.3.1" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.3.1" />