Update oldest supported platform versions

[clarkcb]

Hi, I'm submitting a simple pull request:

The versions for the iOS and tvOS platforms in Package.swift were deprecated so I updated them to .v12.

I ran make lint and found 0 violations, but I did get a number of errors when I ran swift test, most of which appeared to be because a value: label is needed for method parameters, also ones that said a generic parameter 'V' cound not be inferred.

Please let me know if there's anything else you'd like me to do before accepting this.

Thanks, Cary