Bump minimum required Go version for Workhorse to 1.15

We're currently in the process of moving to Go 1.16 across all
distributions, with a minimum required Go version of 1.15. Workhorse is
still using Go 1.13 as the minimum required Go version, which doesn't
make a lot of sense when other components already require a newer
version anyway.

Bump the minimum required version of Go to 1.15 and move over the
default image in CI to 1.16.

.gitlab/ci/workhorse.gitlab-ci.yml

 workhorse:verify:
   extends: .workhorse:rules:workhorse
-  image: ${GITLAB_DEPENDENCY_PROXY}golang:1.15
+  image: ${GITLAB_DEPENDENCY_PROXY}golang:1.16
   stage: test
   needs: []
   script:
@@ -23,14 +23,10 @@ workhorse:verify:
     - apt-get update && apt-get -y install libimage-exiftool-perl
     - make -C workhorse test
 
-workhorse:test using go 1.13:
-  extends: .workhorse:test
-  image: ${GITLAB_DEPENDENCY_PROXY}golang:1.13
-
-workhorse:test using go 1.14:
-  extends: .workhorse:test
-  image: ${GITLAB_DEPENDENCY_PROXY}golang:1.14
-
 workhorse:test using go 1.15:
   extends: .workhorse:test
   image: ${GITLAB_DEPENDENCY_PROXY}golang:1.15
+
+workhorse:test using go 1.16:
+  extends: .workhorse:test
+  image: ${GITLAB_DEPENDENCY_PROXY}golang:1.16

changelogs/unreleased/pks-workhorse-go-1-16.yml

+---
+title: Bump minimum required Go version for workhorse to 1.15
+merge_request: 59347
+author:
+type: other

workhorse/.gitlab-ci.yml

@@ -10,7 +10,7 @@ workflow:
     - if: '$CI_COMMIT_BRANCH =~ /^[\d-]+-stable$/'
 
 default:
-  image: golang:1.13
+  image: golang:1.16
   tags:
     - gitlab-org
 
@@ -41,18 +41,14 @@ changelog:
   - apt-get update && apt-get -y install libimage-exiftool-perl
   - make test
 
-test using go 1.13:
-  extends: .test
-  image: golang:1.13
-
-test using go 1.14:
-  extends: .test
-  image: golang:1.14
-
 test using go 1.15:
   extends: .test
   image: golang:1.15
 
+test using go 1.16:
+  extends: .test
+  image: golang:1.16
+
 test:release:
   rules:
     - if: '$CI_COMMIT_TAG'

workhorse/doc/operations/install.md

 # Installation
 
-To install GitLab Workhorse you need [Go 1.13 or
+To install GitLab Workhorse you need [Go 1.15 or
 newer](https://golang.org/dl) and [GNU
 Make](https://www.gnu.org/software/make/).
 

workhorse/go.mod

 module gitlab.com/gitlab-org/gitlab-workhorse
 
-go 1.13
+go 1.15
 
 require (
 	github.com/Azure/azure-storage-blob-go v0.11.1-0.20201209121048-6df5d9af221d