Bump gitlab-shell version to 13.12.0

This is mainly needed to fix GitPod development since we've upgraded to
using bundler v2.1.4 in the GitLab Development Kit.

GITLAB_SHELL_VERSION

-13.11.0
+13.12.0

changelogs/unreleased/sh-bump-gitlab-shell-13-12.yml

+---
+title: Bump gitlab-shell version to 13.12.0
+merge_request: 47084
+author:
+type: other