Replace <PackageLicenseFile> with <PackageLicenseExpression>

Author: michael-wolfenden

Directory.Build.props

@@ -19,7 +19,7 @@
     <PackageId>EntityFrameworkCore.Exceptions.$(MSBuildProjectName)</PackageId>
     <PackageIcon>Icon.png</PackageIcon>
     <PackageReadmeFile>README.md</PackageReadmeFile>
-    <PackageLicenseFile>License.md</PackageLicenseFile>
+    <PackageLicenseExpression>Apache-2.0</PackageLicenseExpression>
     <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
     <PackageReleaseNotes>Add ConstraintName, ConstraintProperties and SchemaQualifiedTableName to UniqueConstraintException and ReferenceConstraintException</PackageReleaseNotes>