Bump gitlab-shell to v13.7.0

This will make it possible for gitlab-shell to handle relative URLs over
UNIX sockets:
https://gitlab.com/gitlab-org/gitlab-shell/-/merge_requests/406

This unblocks
https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/4498.

GITLAB_SHELL_VERSION

-13.6.0
+13.7.0
\ No newline at end of file

changelogs/unreleased/sh-bump-gitlab-shell-13-7-0.yml

+---
+title: Bump gitlab-shell to v13.7.0
+merge_request: 40132
+author:
+type: fixed