V10: project template (Program.cs and Startup.cs) can be simplified

[tormnator]

I understand it's not possible to go to a minimal hosting model until .Net 7, but I think we can simplify the V10 project template (Program.cs and Startup.cs) to use a couple of the new features in .Net 6:

1. Use top-level-statements in Program.cs
2. Use File-scoped namespace declaration in Startup.cs
3. (Optional), clean up some of the method comments in Startup.cs since they are mostly redundant and don't add much to the code itself.

With these changes the Program.cs would look something like this:

and the Startup.cs would look something like this:

I assume it would be easier to get these changes in before the V10 release, any thoughts?

[tormnator]

Also see #12445 which proposes fully moving to the Minimal Hosting Model.