Remove separate implementations for getting an assembly version when tasks target .NET Core 2.0

[dsplaisted]

Currently there's a separate implementation for .NET Framework and .NET Standard to get the assembly version of a file (FileUtilities.GetAssemblyVersion). On .NET Standard, we use the metadata reader APIs to get this data. AssemblyName.GetAssemblyName has been added to .NET Standard 2.0, so when we retarget our tasks to that, we can get rid of the separate implementation.

[github-actions]

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