Move official build to use VS2019 pool

When moving to 2019 we get the following build error:

```
C:\Program Files\dotnet\sdk\5.0.100-rc.2.20479.15\Sdks\NuGet.Build.Tasks.Pack\build\NuGet.Build.Tasks.Pack.targets(207,5): error NU5128: Some target frameworks declared in the dependencies group of the
nuspec and the lib/ref folder do not have exact matches in the other location. Consult the list of actions below: [G:\NuGet.BuildTasks\src\Microsoft.NuGet.Build.Tasks\Microsoft.NuGet.Build.Tasks.cspro
j]
C:\Program Files\dotnet\sdk\5.0.100-rc.2.20479.15\Sdks\NuGet.Build.Tasks.Pack\build\NuGet.Build.Tasks.Pack.targets(207,5): error NU5128: - Add lib or ref assemblies for the net46 target framework [G:\N
uGet.BuildTasks\src\Microsoft.NuGet.Build.Tasks\Microsoft.NuGet.Build.Tasks.csproj]
##vso[task.logissue type=error;sourcepath=C:\Program Files\dotnet\sdk\5.0.100-rc.2.20479.15\Sdks\NuGet.Build.Tasks.Pack\build\NuGet.Build.Tasks.Pack.targets;linenumber=207;columnnumber=5;code=NU5128;]Some target frameworks declared in the dependencies group of the nuspec and the lib/ref folder do not have exact matches in the other location. Consult the list of actions below:%0D%0A- Add lib or ref assemblies for the net46 target framework
```