There were a couple new issues that were caused by Gitaly absorbing
gitlab-shell functionality:

1. The Gitaly `config.toml` now needs the URL of the GitLab host
(https://gitlab.com/gitlab-org/gitaly/-/merge_requests/2168).

2. `.gitlab_shell_secret` is now read by Gitaly at startup
(https://gitlab.com/gitlab-org/gitaly/-/merge_requests/2214).

3. Praefect SQL failover is now enabled by default
(https://gitlab.com/gitlab-org/gitaly/-/merge_requests/2218). We can
disable this for now since we don't need this for the GitLab tests.

Closes https://gitlab.com/gitlab-org/gitaly/-/issues/2831