Use `builds.dotnet.microsoft.com` not `download.visualstudio.microsoft.com`

`releases.json` and `release.json` files currently reference the VS CDN. That's a bad practice since it requires that both `builds.dotnet.microsoft.com` and `download.visualstudio.microsoft.com` are up. Is either is down, then users cannot download files via this flow. That naturally means that the JSON files should reference files on the `builds.dotnet.microsoft.com` CDN. Also, the VS CDN doesn't have stable URLs, making it impossible to have a stable link to the JSON files should we want to locate them there instead.

Example JSON file:

https://github.com/dotnet/core/blob/218ef74621d76d73c0a562893df875d37ce928ad/release-notes/8.0/releases.json#L23-L33

We are currently validating that the Azure Front Door and Akamai CDNs that we'll be using provide good latency/service to various large countries.

This change also means that we will no longer be publishing new URLs for the VS CDN. It will be used as a private implementation detail for .NET delivery via Visual Studio. All users will need to move to using the dotnet CDN.

FYI: We're going to call `builds.dotnet.microsoft.com` the "dotnet CDN". Feel free to use that terminology. We intend for it to have a very long future.