golang: +go1.17 -go1.15

Go1.17 is incremental improvement over Go1.16 with better and faster compiler,
runtime, module-mode improvements and faster generated code:

https://blog.golang.org/go1.17
https://golang.org/doc/go1.17

Drop support for Go1.15, as that release reached EOL and nothing currently uses
it in SlapOS.

Don't drop support for Go1.12 yet, as that long-ago-EOL and no longer supported
Go release is still being used by software/gitlab.

Remain default at Go1.16 yet.

Switch helloworld to Go1.17 and test this patch on that
software-release.
3 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

00:10:13

passed SlapOS.Eggs.UnitTest-Master.Python3

00:35:16

passed SlapOS.SlapRunner.ResilienceTest-Master

00:21:48