Move from XliffTasks to Microsoft.DotNet.XliffTasks

[premun]

The package name was changed to start with a reserved name (dotnet/xliff-tasks#412).

I tried to update the package name in all repositories that use it but I hit a wall in roslyn-tools as there's some legacy package flow that I don't understand fully so I would leave it up to repo owners.

The new package is here (same feed, prefixed name):
https://dev.azure.com/dnceng/public/_packaging?_a=package&feed=dotnet-eng&package=Microsoft.DotNet.XliffTasks&protocolType=NuGet

[akoeplinger]

This was done by #1058