Update naming of templates

[grokys]

Describe the bug

We still use the outdated ".NET core" naming in our templates, e.g.

https://github.com/AvaloniaUI/avalonia-dotnet-templates/blob/master/templates/csharp/app/.template.config/template.json#L8

This should just be .NET now I believe.

To Reproduce

Template Name                        Short Name                 Language  Tags
-----------------------------------  -------------------------  --------  -----------------------------------------
Avalonia .NET Core App               avalonia.app               [C#],F#   Desktop/Xaml/Avalonia/Windows/Linux/macOS
Avalonia .NET Core MVVM App          avalonia.mvvm              [C#],F#   Desktop/Xaml/Avalonia/Windows/Linux/macOS

Avalonia Templates version

master

Avalonia version

all

Additional context

No response