Pull mirroring using SSH key fails on Staging

Summary

Pull mirroring using SSH key fails on Staging.

This is causing a QA test to fail: #35152 (closed)

It's not clear how long it's been failing. At least since Oct 14 when the test ran properly for the first time: https://ops.gitlab.net/gitlab-org/quality/staging/-/jobs/674796

Steps to reproduce

1. On Staging, create a project to be the mirror source.
2. Configure the target project to pull mirror from the source project using SSH (being sure to detect host keys).
3. Copy the SSH key and add it to the mirror source as a deploy key.
4. Update the mirroring in the mirror target.

Example Project

https://staging.gitlab.com/mlapierre/mirror-target

https://staging.gitlab.com/mlapierre/mirror-source

What is the current bug behavior?

Mirroring fails with an error:

2:Fetching remote upstream failed:  connect to host staging.gitlab.com port 22: Connection timed out
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

Sentry error log: https://sentry.gitlab.net/gitlab/staginggitlabcom/issues/1035840/

What is the expected correct behavior?

Mirroring succeeds

Output of checks

This bug happens on staging.gitlab.com