build: bump go to 1.16

What does this MR do?

go 1.17 was released yesterday.

Release notes: https://golang.org/doc/go1.17

go 1.15 is now EOL and it can be dropped safely once gitlab 14.2 is released (according to the current policy: https://docs.gitlab.com/ee/development/go_guide/#supporting-multiple-go-versions)

status from https://gitlab.com/feistel/canwebumpgoyet:

go version (latest):     1.17.0
go version (additional): 1.16.7

Gitlab Version: master
CNG version:                    master      (go 1.16.4)
Omnibus-builder image revision: 1.6.0       (go 1.16.4)

Gitlab Version: v14.1.0-ee
CNG version:                    v14.1.0-ee  (go 1.16.4)
Omnibus-builder image revision: 1.6.0       (go 1.16.4)

Gitlab Version: v14.0.0-ee
CNG version:                    v14.0.0-ee  (go 1.16.4)
Omnibus-builder image revision: 1.6.0       (go 1.16.4)

Gitlab Version: v13.12.0-ee
CNG version:                    v13.12.0-ee (go 1.16.3)
Omnibus-builder image revision: 1.2.0       (go 1.15.8)

TODO

• I added the Changelog trailer (e.g. Changelog: feature) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines