Update Nuget package license format to new format

[tsimbalar]

What issue does this PR address?
#1265

[tsimbalar]

For some reason, the Nuget package generated by https://ci.appveyor.com/project/serilog/serilog/builds/21388982/artifacts does not seem to have the proper licensing information

Not sure what to think of this ... it worked fine with Serilog.Sinks.Console and Serilog.Settings.Configuration.

[nblumhardt]

🤔

This repository does include a global.json file that pins the tooling to 2.1, while Console and Configuration don't - perhaps shifting the tooling version forward will do the job?

[nblumhardt]

Any thoughts on what we should do with this one, @tsimbalar ?

[tsimbalar]

Thanks for the nudge !

Sorry, I haven't been able to do much Serlog work lately 😑.

I'll try to get to it eventually, but if somebody has the time to look into it, go for it !

[nblumhardt]

@tsimbalar I'm only just getting some time put aside again, too. Looking forward to digging into some more substantial Serilog features in the next little while, if the calendar stays sane (fingers crossed :-))

I'll take a look if no one else beats me to it.

[nblumhardt]

Works now that #1315 has updated the build-time SDK 🎉