Use the new license expressions (#2878)

source/SkiaSharp.NuGet.props

@@ -18,7 +18,7 @@
     <Authors>Microsoft</Authors>
     <Copyright>© Microsoft Corporation. All rights reserved.</Copyright>
     <PackageRequireLicenseAcceptance>true</PackageRequireLicenseAcceptance>
-    <PackageLicenseFile>LICENSE.txt</PackageLicenseFile>
+    <PackageLicenseExpression>MIT</PackageLicenseExpression>
 
     <NoWarn>$(NoWarn);NU5125;NU5048</NoWarn>