Bump minimum iOS requirement to iOS 11

I think we should bump the minimum iOS version from iOS 10 to iOS 11 for the 1.18 release.

This would speed up iOS builds a lot because we can (or rather have to) drop armv7 and x86 builds from the set of architectures to build.

Apparently it will also be needed/advantageous for vulkan zerocopy for IOSurface usage.

According to this site with iOS version statistics 10.x is in use by only 1.4% of devices, and bumping to 11.x will still cover about 97% of devices, probably more by the time 1.18 gets released.

If I'm reading the table right this would mean effectively dropping support for iphone 5c, iphone 5, ipad 4G (7 year old models).

I think we should do this, 1.4% is not worth it, and 97% is pretty much "everything non-ancient" still.