golang: Switch default to Go1.17

Go1.17 is current stable Go release which, besides other improvements,
compared to Go1.16 generates code that runs 5-10-15% faster(*):

https://go.dev/blog/go1.17

It makes sense to use that version by default.

Cc'ing people who might be affected:

- caddy   (@luke)
- hugo    (@lu.xu)
- repman, restic  (@alain.takoudjou, @jerome)
- galene  (@tomo, @alain.takoudjou)
- gitlab  (@alain.takoudjou, @jerome, @lpgeneau)
- grafana (@jerome)
- theia   (@jerome, @xavier_thompson)

If for some reason Go1.17 default does not work for a software,
maintainer of that software release - please pin Go to Go1.16
explicitly inside that particular software-release.

(*) Go blog mentions 5% speedup on the average, but it was reported that
    sometimes for important inner loops the speedup can be as much as 15%.

/reviewed-by @jerome
/reviewed-on nexedi/slapos!1082