build: Bump go version to 1.16

What does this MR do?

Bump workhorse go version to 1.16.

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

status:

go version (latest):     1.17.0
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: v14.2.0-ee
CNG version:                    v14.2.0-ee  (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)

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no code/feature changes

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.