Update GitLab to 18.0

According to the maintenance policy, GitLab should release the next major version towards the end of May of next year.

Because of the apt-pinning configuration implemented in #40769 (closed) (closed), unattended-updates will not automatically install this update.

This is a reminder ticket for the planning, announcement and execution work that must be happen manually.

A new ticket like this should be created with a due date 12 months later when this ticket is due.

update, the plan is:

1. pg16 upgrade
2. gitlab 18 upgrade
3. rerun pgbackrest backup: fab -H gitlab-02.torproject.org postgresql.backup --no-wait --backup-server="bungei.torproject.org" --kind=diff
4. postgresql 17 upgrade: fab --dry -H gitlab-02.torproject.org postgresql.upgrade --backup-server="bungei.torproject.org"
5. trixie upgrade (hold until https://gitlab.com/gitlab-org/gitlab/-/issues/548937 ?)