Replace mention of .NET Core 3 to .NET 6

[kant2002]

That gives very stale look on the WPF. Designer is working too.

[vishalmsft]

@kant2002 Is there a reason to remove the "Core" word? Removing "3" is really great and thanks for taking the efforts to update the stale documents.

[kant2002]

My idea is that “.Net” about 2 year like new name. There still a bit of confusion between what is .Net and what’s difference with .Net Core. So I think better use “current” name if project is not freezed in time. And WPF has “perception” of being abandoned. So having Net Core as primary name will just strengthens this opinion. Which I do not think justified

[ThomasGoulet73]

They removed the "Core" suffix in the Windows Forms repo (getting-started.md in dotnet/winforms). IIRC the "Core" suffix is only supposed to be used when comparing .Net Core and .Net Framework (Exemple: ".Net Core supports XYZ while .Net Framework does not.").

[kant2002]

You see 😄 here we have perfect example of confusion (or simplificaiton).
We have 2 lineages and 3 names.
Lineage CLR

• .NET Framework 4.x

Lineage CoreCLR

• .NET Core 1.x-3.x
• NET > 5

I take names of CLR and CoreCLR liberally here.
When person reach docs on the GitHub I want put emphasis that this repo is for development of recent versions. This will never give impression that this is most loved framework, but I believe that you still care about it. .NET Core for anybody active in development is red flag that this is abandonware. Legacy is unfashionable and ostracized by community. WPF did not deserve that.