Check if installation of .NET Core is still needed in travis-ci

[nils-a]

While working on #704 I stumbled over the current documentation for travis-ci.

It contains the following lines:

before_install:
  - choco install dotnetcore-sdk -version 3.1.402

The question is: Is the installation if .NET Core still a requirement?
If so, should we update the version (current would be 3.1.404)?
Could the installation also be done using the dotnet matrix key?

[AdmiringWorm]

Looking at https://docs.travis-ci.com/user/reference/windows/ it is still required.
.NET Core is not pre-installed for the image, and the language csharp is not supported, as such it must be installed through chocolatey.

[nils-a]

So everything remaining would be to update the version to the current 3.1.404?
(Or switch to installing dotnetcore-3.1-sdk without the --version ?)

[AdmiringWorm]

I think installing the dotnetcore-3.1-sdk would be appropriate.
No need to throw in any version arguments when it isn't necessary.

May also want to throw in a note about the runtimes as well, as only installing the sdk package won't magically allow .NET Core apps to run.

• dotnetcore-3.1-runtime -> For console applications (would be required for unit tests as well I believe)
• dotnetcore-3.1-desktopruntime -> For windows GUI/Desktop applications
• dotnetcore-3.1-aspnetruntime -> For ASP.NET Core applications
• Or alternatively dotnetcore-runtime --version 3.1.10 for all of the above (except the SDK)