Remove ref reference to System.Security.Cryptography.Cng

[David-Engel]

After removing support for .NET Standard, references to System.Security.Cryptography.Cng were removed but one was left in the ref project:

SqlClient/src/Microsoft.Data.SqlClient/netcore/ref/Microsoft.Data.SqlClient.csproj

Line 21 in 689cc25

<PackageReference Include="System.Security.Cryptography.Cng" Version="$(SystemSecurityCryptographyCngVersion)" />

This should be removed.

Once done, #2048 can be closed.

[ErikEJ]

@David-Engel I will have a look