This upgrade fixes a problem that caused a single listener to monopolize
a Unicorn worker. By default, GitLab Omnibus configures Unicorn to
listen both on a UNIX domain socket and a TCP socket. One of those
sockets can be starved if many requests are queued
(https://yhbt.net/unicorn-public/20200416092457.GA5000@dcvr/).

Full list of changes:
https://github.com/defunkt/unicorn/compare/v5.4.1...v5.5.5

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/213257