Switch to using PackageLicenseExpression (#668)

Author: lahma

src/Directory.Build.props

@@ -15,7 +15,7 @@
         <IncludeSymbols>true</IncludeSymbols>
         <EmbedUntrackedSources>true</EmbedUntrackedSources>
         <AllowedOutputExtensionsInPackageBuildOutputFolder>$(AllowedOutputExtensionsInPackageBuildOutputFolder);.pdb</AllowedOutputExtensionsInPackageBuildOutputFolder>
-        <PackageLicenseFile>LICENSE</PackageLicenseFile>
+        <PackageLicenseExpression>MIT</PackageLicenseExpression>
         <PackageIcon>icon.png</PackageIcon>
     </PropertyGroup>