Update license and icon info in nuget packages

[tusmester]

The following tags are deprecated in a nuget package. We should convert our packages to use the new tags.

• licenseUrl - new tag: license
• iconUrl - new tag: icon

More info:

• https://docs.microsoft.com/en-us/nuget/reference/nuspec#license

Todo

• [ ] Choose the appropriate identifier (GPL2, but which one): https://spdx.org/licenses/
• Update packages gradually, as we release them.

Most likely this one: GPL-2.0-only https://spdx.org/licenses/GPL-2.0-only.html