Thank you, I hope the project is of some use for the community.
To answer your question the script is using the locally installed MonoGame templates. So it is using 3.8 or newer depending what is installed.
To install / update the dotnet CLI templates you only need one command
dotnet new --install MonoGame.Templates.CSharp
MonoGame Kickstarter checks at some point if the MonoGame templates are installed since they are required and prints the command if none are found.
The command is from the official MonoGame documentation.
Since MonoGame Kickstarter is at the core only just using the dotnet CLI you can modify and add or remove anything you want later from the generated solution and / or projects.