Update Infrastructure (#961)

* Update form .NET Core 2.2 to .NET Core 3.1

- Global variable for projects
- Change TFMs from netcoreapp2.2 to netcoreapp3.1
- Update global.json
- Update build.json
- Remove direct framework cake TestCore task
- Update travis dotnet version
- Update azure pipline file
- Update vscode launch.json

* Add Central Package Management

* Fix xUnit Breaking Change for MemberData type

* Fix xUnit breaking change for duplicate test method name

* Fix Rang/Index type conflict with System.Rang/Index

* Update vscode tasks.json

* Change serviceHostExecutable path in ServiceTestDriver.cs

* Downgrade SDK version (https://github.com/appveyor/ci/issues/3440)

- Appveyor hasn't installed latest SDK therefore I downgrade it until they install it.

* Dump Microsoft.SqlServer.DACFx

test/Microsoft.SqlTools.Hosting.UnitTests/Microsoft.SqlTools.Hosting.UnitTests.csproj

@@ -1,14 +1,14 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>netcoreapp2.2</TargetFramework>
+    <TargetFramework>$(TestProjectsTargetFramework)</TargetFramework>
     <IsPackable>false</IsPackable>
 	<TargetLatestRuntimePatch>true</TargetLatestRuntimePatch>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.3.0" />
-    <PackageReference Include="Moq" Version="4.8.2" />
-    <PackageReference Include="xunit" Version="2.3.1" />
-    <PackageReference Include="xunit.runner.visualstudio" Version="2.2.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" />
+    <PackageReference Include="Moq"/>
+    <PackageReference Include="xunit"/>
+    <PackageReference Include="xunit.runner.visualstudio" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\src\Microsoft.SqlTools.DataProtocol.Contracts\Microsoft.SqlTools.DataProtocol.Contracts.csproj" />