Merge branch 'ashmckenzie/use-golang-1-21' into 'main'

Bump GO_VERSION to 1.21

See merge request https://gitlab.com/gitlab-org/gitlab-shell/-/merge_requests/898



Merged-by: Igor Drozdov <idrozdov@gitlab.com>
Co-authored-by: Ash McKenzie <amckenzie@gitlab.com>

.gitlab-ci.yml

@@ -16,7 +16,7 @@ variables:
   TRANSFER_METER_FREQUENCY: "1s"
   DOCKER_VERSION: "20.10.15"
   BUNDLE_FROZEN: "true"
-  GO_VERSION: "golang-1.20"
+  GO_VERSION: "golang-1.21"
   GOPATH: $CI_PROJECT_DIR/.GOPATH
   DEBIAN_VERSION: "bullseye"
   RUBY_VERSION: "ruby-3.2"