LICENSE file is required by nuget pack

This error showed up in the internal CI when packing the PR build:

```
/usr/share/dotnet/sdk/3.1.102/Sdks/NuGet.Build.Tasks.Pack/buildCrossTargeting/NuGet.Build.Tasks.Pack.targets(198,5): error NU5030: The license file 'LICENSE' does not exist in the package. [/tmp/build/db7f1294/rabbitmq_dotnet_client/projects/client/RabbitMQ.Client/RabbitMQ.Client.csproj]
```

Author: lukebakken

projects/client/RabbitMQ.Client/RabbitMQ.Client.csproj

@@ -38,6 +38,10 @@
     <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleTo">
       <_Parameter1>Unit, PublicKey=00240000048000009400000006020000002400005253413100040000010001008d20ec856aeeb8c3153a77faa2d80e6e43b5db93224a20cc7ae384f65f142e89730e2ff0fcc5d578bbe96fa98a7196c77329efdee4579b3814c0789e5a39b51df6edd75b602a33ceabdfcf19a3feb832f31d8254168cd7ba5700dfbca301fbf8db614ba41ba18474de0a5f4c2d51c995bc3636c641c8cbe76f45717bfcb943b5</_Parameter1>
     </AssemblyAttribute>
+    <None Include="..\..\..\LICENSE">
+      <Pack>True</Pack>
+      <PackagePath></PackagePath>
+    </None>
     <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0" PrivateAssets="All" />
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0" PrivateAssets="All" />
     <PackageReference Include="MinVer" Version="2.2.0">