Update templates to target .NET 8 once it's been more widely adopted

[justinvp]

Our .NET templates currently target .NET 6 (the previous only LTS version). Earlier this week, .NET 8 was released, the new LTS version. We should consider upgrading our templates to target .NET 8, so new projects have access to all the new .NET 8 features by default, once .NET 8 has been more widely adopted.

According to https://dotnet.microsoft.com/en-us/download/dotnet:

• .NET 6 will no longer be supported on November 12, 2024
• .NET 7 (which is not an LTS version) will no longer be supported May 14, 2024

Perhaps around May 14, 2024 would be a good time to move from targeting .NET 6 to .NET 8. We will definitely want to do it ahead of .NET 6 no longer being supported November 12, 2024.