Update Nuget package license format to new format

[tsimbalar]

licenseUrl is now deprecated so we use a license expression

Tracked in serilog/serilog#1265

This properly populate the license information in the nuget package :

[skomis-mm]

👍

Would be useful to also check license type=File instead (because we have https://github.com/serilog/serilog-settings-configuration/blob/dev/LICENSE file) for having less reasons to sync them (unlikely though :)).

[tsimbalar]

I think which approach we choose does not matter too much as long as we are consistent across the Serilog repos :)