build: bump go to 1.15
go 1.13 is EOL.
Bump go version according to gitlab policy: https://docs.gitlab.com/ee/development/go_guide/#supporting-multiple-go-versions
Status:
go version (latest): 1.16.7
go version (additional): 1.15.15
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)