The azureedge.net domain might stop working as detailed in dotnet/core#9671

Changes in this PR:

dotnet: update the logic to determine the latest version
dotnet and oryx: copy latest dotnet-install script from
https://github.com/dotnet/install-scripts/blob/main/src/dotnet-install.sh
(Normally this happens automatically by a GitHub Action, but there is a sense of urgency here, so I did it manually)
Additional context

https://devblogs.microsoft.com/dotnet/critical-dotnet-install-links-are-changing/
https://build5nines.com/retirement-of-azureedge-net-dns-edg-io-business-closure-and-what-you-need-to-know/