Update the documentation about the templates on learn.microsoft.com for .NET 7

learn.microsoft.com features 2 articles where the templates are mentioned:

• https://learn.microsoft.com/en-us/dotnet/core/tools/dotnet-new#arguments - list of the available templates
• https://learn.microsoft.com/en-us/dotnet/core/tools/dotnet-new-sdk-templates - the documentation on the templates available with .NET SDK, including specific options.

Consider updating the articles for .NET 7:

• in case new templates were added, please them add them to the table list
• in case the template options were added, modified or removed, please adapt the description accordingly

Note that for many cases, the EOL frameworks are present: netcoreapp3.0, net5.0 etc. Consider removing them.

The source code is here:

• https://github.com/dotnet/docs/blob/main/docs/core/tools/dotnet-new.md
• https://github.com/dotnet/docs/blob/main/docs/core/tools/dotnet-new-sdk-templates.md

[tdykstra]

is it ok to keep old versions in Introduced column?

Yes, I would keep them there.

[dipeshmsft]

@vlada-shubina, are there some docs mentioning the --nullable flag?