Update templating text to use @ instead of :: symbol

[marcpopMSFT]

The package::version format is the legacy format and the recommendation now is to use @ instead. We have an SDK PR to update this but some of the strings come from the template engine as well. Both repos already support the new format so this is just a matter of ensuring all messaging recommends customers use the new format.

Here's the SDK PR: https://github.com/dotnet/sdk/pull/51153

For the templating repo:

• Update InstallRequest.DisplayName
• Update TemplatePackage.DisplayName properties
• identify any additional changes needed to strings or tests related to this transition

Both DisplayNames come from the Microsoft.TemplateEngine.Edge package