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.
CHANGELOG.md:
v13.12.0
- Upgrade Bundler from 1.17.2 to 2.1.4 !428
- Log Content-Length bytes in API response !427
- Bump default Ruby version to v2.7.2 !426
Edited by Stan Hu