Proposal: Raise minimal supported Go version with upcoming 0.14.0 release

Currently (as of 0.13.0), the minimal supported Go version is 1.10.8, which is the latest point release in 1.10.x (released on 2019-01-23). I propose that we raise this version with 0.14.0 release. There are 3 potential versions we can raise it to:

1. go1.12.17 (released on 2020-02-12). This proposal is based on the release cycles of the 2 projects. (Go has a release very 6 months, while thrift has a release roughly every 12 months, so we raise 2 versions of Go with our release). This would give us benefit of proper go modules support, which was introduced in go 1.11.
2. go1.13.15 (released on 2020-08-06). This would give us the additional benefit of better error/exception handling, as currently in go library code there are a few places we have to do extra work because we have to support pre-go1.13 (examples: 1, 2, 3)
3. go1.14.x. There's no additional benefit for this one, but this will be the latest minimal officially supported go release when we release 0.14.0 (as stated by Go release policy, only 2 most recent major releases will be supported by security fixes).

I don't know if any of the thrift users are stuck with an older Go version. Consider go has very strong backward compatibility guarantee on the whole 1.x tree, I highly doubt that would be the case (unless someone is stuck on a platform that's dropped in later Go releases).