Replace <PackageLicenseUrl /> with <PackageLicenseExpression />

Closes #1973

Author: hazzik

build-common/NHibernate.props

@@ -24,7 +24,7 @@
     <PackageProjectUrl>http://nhibernate.info</PackageProjectUrl>
     <PackageIconUrl>https://raw.githubusercontent.com/nhibernate/nhibernate-core/master/logo/NHibernate-NuGet.png</PackageIconUrl>
     <PackageRequireLicenseAcceptance>false</PackageRequireLicenseAcceptance>
-    <PackageLicenseUrl>https://raw.githubusercontent.com/nhibernate/nhibernate-core/master/LICENSE.txt</PackageLicenseUrl>
+    <PackageLicenseExpression>LGPL-2.1-only</PackageLicenseExpression>
     <PackageReleaseNotes>https://github.com/nhibernate/nhibernate-core/blob/$(VersionPrefix)/releasenotes.txt</PackageReleaseNotes>
     <RepositoryUrl>https://github.com/nhibernate/nhibernate-core.git</RepositoryUrl>
     <RepositoryType>git</RepositoryType>