Src-cli latest version endpoint is unauthenticated and can fall back to different version

Created by: eseliger

See https://sourcegraph.slack.com/archives/CMMTWQQ49/p1658873113895129, this endpoint uses unauthenticated requests to github which occasionally get rate-limited and thus we fall back to the minimumversion const. This is not necessarily problematic, but it causes us to serve different versions at various times during the day (at the very least the const, but not always the actually latest patch version)