Drop Go v1.12 support

We officially use Go v1.13+ for everything
(https://docs.gitlab.com/ee/install/requirements.html#go-versions), and
dropping Go v1.12 allows us to use new methods such as
`http.NewRequestWithContext`.

.gitlab-ci.yml

@@ -10,7 +10,7 @@ workflow:
     - if: '$CI_COMMIT_BRANCH =~ /^[\d-]+-stable$/'
 
 default:
-  image: golang:1.12
+  image: golang:1.13
   tags:
     - gitlab-org
 
@@ -35,10 +35,6 @@ verify:
   - apt-get update && apt-get -y install libimage-exiftool-perl
   - make test
 
-test using go 1.12:
-  extends: .test
-  image: golang:1.12
-
 test using go 1.13:
   extends: .test
   image: golang:1.13

changelogs/unreleased/id-drop-go-1-12.yml

+---
+title: Drop Go v1.12 support
+merge_request: 532
+author:
+type: changed