Define supported Go versions

The SDK currently doesn't have a policy regarding supported Go versions. We have been supporting Go 1.11 for a while even Go team doesn't support the version.

Right now, the motivation tends to come from dependencies. If they are having vulnerabilities and we want to upgrade them, we tend to realize that newer dependencies don't support older Go versions.

We probably should have a version supporting policy upfront, instead of deciding case-by-case basis. If supporting old Go versions have some value, we may be okay to ask our dependencies to support them.