Thanks. FYI, I took the liberty to edit your response to use code block formatting for better readability, following Community, first steps: Code, config, log block formatting in topics and replies (linked from the FAQ).
The SSH layer is provided by the container image operating system, not the GitLab application itself. You would need to inject a custom ssh_config for the server, like your topic title suggests. This needs to be done at build time, meaning to create your own custom image, with an override for /assets/sshd_config in the Dockerfile.
I found this feature proposal to make it configurable at runtime: Allow inclusion of user sshd configuration files in docker (#8124) · Issues · GitLab.org / omnibus-gitlab · GitLab Suggest commenting there, and upvoting.
An alternative option is to not expose SSH, and only use HTTPS for all Git operations. That eliminates the SSH TLS algorithm risk.