Update project to use SPDX LicenseExpression (resolve warning NU5125: The 'licenseUrl' element will be deprecated)

Author: augustoproiete

src/Serilog.Sinks.File/Serilog.Sinks.File.csproj

@@ -12,7 +12,7 @@
     <PackageTags>serilog;file</PackageTags>
     <PackageIconUrl>http://serilog.net/images/serilog-sink-nuget.png</PackageIconUrl>
     <PackageProjectUrl>http://serilog.net</PackageProjectUrl>
-    <PackageLicenseUrl>http://www.apache.org/licenses/LICENSE-2.0</PackageLicenseUrl>
+    <PackageLicenseExpression>Apache-2.0</PackageLicenseExpression>
     <RepositoryUrl>https://github.com/serilog/serilog-sinks-file</RepositoryUrl>
     <RepositoryType>git</RepositoryType>
     <GenerateAssemblyVersionAttribute>false</GenerateAssemblyVersionAttribute>