Move extensions to Microsoft.Extensions.DependencyInjection namespace

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As this is the recommendation nowadays

https://docs.microsoft.com/en-us/aspnet/core/fundamentals/dependency-injection?view=aspnetcore-3.1

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e.g. LoadConfigurationFromSection , AddNLogWeb

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I really should create a PR for this :)

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We have already 3 namespaces

using NLog;
using NLog.Extensions.Logging;
using NLog.Extensions.Hosting;

and we could not move them all to Microsoft.Extensions.DependencyInjection, so this isn't really the way