Remove upper bound on some System.* nugets

[pauldendulk]

This is an attempt to remedy this: #2324. But there are likely to be other limiting packages.

I only looked at the four System.* packages, because it is pretty save there are no breaking changes there.

• System.Drawing.Common - Removed upper bound
• System.Text.Encoding.CodePages - Removed upper bound
• System.Linq.Async - Remove entirely because was not used
• System.Memory - leave, it is a very specific workaround for some issue on iOS. Can be removed in master.

I don't have a general strategy wrt upper bounds yet. Two things to take into account:

• If we test with one version we don't want to allow just any other version
• If it is unlikely to introduce breaking changes we should allow higher major version.

Let's see where the next problem is.