Update upstream CDN URLs because Edgio went bankrupt

[Aldaviva]

https://www.bleepingcomputer.com/news/microsoft/microsoft-issues-urgent-dev-warning-to-update-net-installer-link/

dotnet/core#9671

https://devblogs.microsoft.com/dotnet/critical-dotnet-install-links-are-changing/

[Aldaviva]

This project was not using any of the affected Azure CDN by Edgio URLs.

It was using the .NET releases index file in Azure Blob Storage, along with any URLs which that file points to (currently https://dotnetcli.blob.core.windows.net/dotnet/...). This URL has been changed to use the Azure Traffic Manager domain builds.dotnet.microsoft.com for better performance, although the old URL would have continued working. All other upstream URLs used by this project are controlled by the contents of https://builds.dotnet.microsoft.com/dotnet/release-metadata/releases-index.json (including transitively). Currently, this includes the domains dotnetcli.blob.core.windows.net and download.visualstudio.microsoft.com, neither of which are affected.