golang: v↑ go1.16

Go1.16 is incremental improvement over Go1.15 with better and faster compiler,
runtime and module-mode improvements:

https://blog.golang.org/go1.16
https://golang.org/doc/go1.16

Since this Go release switches GO111MODULE default from "auto" to "on", but we
still have in-tree GOPATH-mode uses, add GO111MODULE=auto to our goenv.sh to
preserve previous behaviour for now.

Don't drop support for Go1.14, Go1.13, Go1.12 yet, as that no longer supported
Go releases are still being used by in-tree components(*).

Remain default at Go1.15 yet.

Switch helloworld to Go1.16 and test this patch on that
software-release.

(*) theia, caddy, galene, replication-manager, restic, gitlab and grafana