Update GITLAB_SHELL_VERSION to 3.6.3
Because VERSION at the gitlab-shell v3.6.2 tag was still 3.6.1: https://gitlab.com/gitlab-org/gitlab-shell/blob/v3.6.2/VERSION (my bad!)
VERSION at the v3.6.3 tag is now 3.6.3: https://gitlab.com/gitlab-org/gitlab-shell/blob/v3.6.3/VERSION