Clarify Golang version support policy

We say in the README that tools can be built with v1.9 or later. Actually, we don't support v1.9 (see https://github.com/mongodb/mongo-tools/pull/176). That PR is still relevant since Azure still only supports Go 1.9 (https://docs.microsoft.com/en-us/azure/cloud-shell/features#language-support). I don't think we should continue to support Go 1.9 since it would require us maintaining our own version of pkix.RDNSequence.String().

I think we should only officially support currently supported versions of Go. Currently that's just 1.13 and 1.14.