[nuget] Hide old betas

[imnasnainaec]

https://github.com/sillsdev/libpalaso/wiki/Release-Procedure#additional-necessary-actions says

Delete (hide) the pre-release versions on nuget.org (e.g. once version 8.0.0 is released we should hide the pre-release version 8.0.0-pre-whatever to remove clutter). SIL.NuGetCleaner can be used to do that.

But there are still betas available for 14.0.1 through 16.0.0.