nuget `licenseUrl` depreciated

[mhowlett]

nuget package licenseUrl metadata is depreciated. we need to switch over to using license instead: https://docs.microsoft.com/en-us/nuget/reference/nuspec#license

[ThomasTosik]

Would be nice. More and more people are using SBOM generation and CVE/Policy scanners. And this lib does not properly fit into this.

[anchitj]

https://github.com/confluentinc/confluent-kafka-dotnet/pull/2192/files Should fix