Update gitlab-shell to v13.6.0

This adds support obtaining personal access tokens via SSH (https://gitlab.com/gitlab-org/gitlab-shell/-/merge_requests/397)

GITLAB_SHELL_VERSION

-13.5.0
+13.6.0
\ No newline at end of file

changelogs/unreleased/pb-gitlab-shell-13-6-0.yml

+---
+title: Update gitlab-shell to v13.6.0
+merge_request: 39675
+author:
+type: added