vscode-extensions.ms-dotnettools.csharp: fix license.

Initially the C# extension was fully open source and had the MIT license, however, after changing to the Roslyn language server and adding the vsdbg binary, it has switched to a non-free license. It even has builtin guards against running on non-microsoft VSCode builds now.
Some things in the C# extension can be built from source, but the debugging layer is fully based on vsdbg which is part of Visual Studio which is proprietary, so the extension as a whole is unfree because of it.

Author: GGG-KILLER

pkgs/applications/editors/vscode/extensions/ms-dotnettools.csharp/default.nix

@@ -139,7 +139,7 @@ vscode-utils.buildVscodeMarketplaceExtension {
   meta = {
     description = "Official C# support for Visual Studio Code";
     homepage = "https://github.com/dotnet/vscode-csharp";
-    license = lib.licenses.mit;
+    license = lib.licenses.unfree;
     maintainers = with lib.maintainers; [ ggg ];
     platforms = [
       "x86_64-linux"