Update dependency from Microsoft.Extensions.Configuration.AzureKeyVault to Microsoft.IdentityModel.Clients.ActiveDirectory to version 3.13.9

[DamianEdwards]

The version of Microsoft.IdentityModel.Clients.ActiveDirectory that Microsoft.Extensions.Configuration.AzureKeyVault is depending on is old and points to an incoherent package that is now causing NuGet restore warnings. We need to update to version 3.13.9 of Microsoft.IdentityModel.Clients.ActiveDirectory which now depends on NETStandard.Library directly and thus gets System.Net.Http from there (rather than asking for a non-existent version).

Microsoft.IdentityModel.Clients.ActiveDirectory 3.13.5 depends on System.Net.Http (>= 4.0.1) but System.Net.Http 4.0.1 was not found. An approximate best match of System.Net.Http 4.1.0 was resolved.
Microsoft.IdentityModel.Clients.ActiveDirectory 3.13.5 depends on System.Text.RegularExpressions (>= 4.0.12) but System.Text.RegularExpressions 4.0.12 was not found. An approximate best match of System.Text.RegularExpressions 4.1.0 was resolved.

[DamianEdwards]

FYI @muratg @mikeharder

[muratg]

Assigning to @pakrym

[mikeharder]

Should this be Preview2?

[muratg]

@mikeharder Good question - I guess if we can get to it we probably should.

cc @Eilon

[DamianEdwards]

If we can we should