LicenseUrl is deprecated

[xperiandri]

https://github.com/NuGet/Home/wiki/Packaging-License-within-the-nupkg

Looks like you need to change it to <PackageLicenseExpression>Apache-2.0</PackageLicenseExpression> in packages.props

[martincostello]

Updated in 790d017 as part of #280.

[martincostello]

Fixed by d03ee2b in #290.