Include Copyright notice in Nuget packages

[peter-durrant2]

Add copyright information to nuspec files for SBOM scanning tools to read.

Add <Copyright>Copyright (c) 2016, NLog</Copyright> to .csproj files.

[snakefoot]

Created #747 to add <copyright>-details.

[snakefoot]

NLog.Extensions.Logging ver. 5.3.12 has been released:

• https://www.nuget.org/packages/NLog.Extensions.Logging/5.3.12

Should include <copyright>-details.