Use new CDN for dotnet builds

[sliekens]

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/

[sliekens]

Cc @samruddhikhandale @Chuxel @joshspicer @gauravsaini04