Minimum Go version for the driver

[shueybubbles]

It's not easy to maintain backward compatibility while wanting to take advantage of new Go features in the driver. We also need to assure compliance with modern security standards.
EG some basic stuff like using %w in fmt.Errorf can't be widely adopted until at least 1.13.
The Go core itself is only supported for 2 major versions. Meanwhile the appveyor tests have been running with Go versions as low as 1.10.
As part of my addition of Always Encrypted (#116 ) I am going to upgrade our minimum Go version.