Use Microsoft.Extensions.Logging.Abstractions instead of Microsoft.Extensions.Logging to reduce dependencies.

[Summpot]

Fixes #717 because Microsoft.Extensions.Logging.Abstractions has very few dependencies.
#717 appears to have been fixed elsewhere, and now references to higher versions of Microsoft.Extensions.Logging.Abstractions work properly in the Microsoft.Azure.Functions.Worker.Sdk project.

[badrishc]

We need the .nuspec for our CI auto-release based on $version in there, so it should not be removed.

[Summpot]

@badrishc But where are the related scripts?
I noticed that you create a tag in the format vx.x.x when releasing, so I think it would be a better choice to use GitVersion instead of this text template, which automatically reads the version on the tag and generates the corresponding AssemblyInfo.cs.

[badrishc]

We can look into whether using this instead of $version is possible in our internal release pipeline. But generally would rather not fix something that isn't broken.

[Summpot]

The dotnet pack will now produce nuget packages with version 2.5.9+x. After you run git tag v2.5.9, it will produce nuget packages with version 2.5.9.