Update nuget packages

[nagybalint001]

I have updated the nuget packages to the latest versions.

NJsonSchema had some interface changes, I'm not sure that I have replaced everything correctly. I also updated the SignalR version in the Core project, the newer version is part of the aspnet core framework, which can't be referenced in a netstandard project, that's why I changed the targetframeworks. (I couldn't find a workaround to reference the newer version of SignalR without this change.)

The output of the new version is the same, except for this part (undefined):

export interface Person {
    firstName: string | undefined;
    lastName: string | undefined;
}

export interface Event {
    Type: string | undefined;
}

But according to the C# classes this seems OK to me.

[RicoSuter]

Thanks for the PR, first check looks very good.

[RicoSuter]

Looks good. Ill check the open comments

[RicoSuter]

Any updates on this?

[RicoSuter]

Ref: https://github.com/RicoSuter/SigSpec/issues/2#issuecomment-585999397

Cant we use .NET Core 3.0 for more compatibility?

[RicoSuter]

@TomSmith27 are there things here we still need to merge?

[TomSmith27]

Looks like this updates to .NET core 3 which could be useful to do however there are conflicts because of the updates i made

[RicoSuter]

Do you still want to merge this PR? Can you fix the conflicts?