Fix allow mtls header

What does this MR do and why?

Fixes this issue: gitlab-development-kit#2901 (closed)

It's very important to note that this won't solve the linked issue until this MR is merged (which will take some time to complete). So we will have to keep the Topology Service version pinned for GDK until then.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Checkout this branch in the monolith
2. rm -fr gitlab-cells and enable cells as per https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/cells.md
3. Run gdk reconfigure && gdk cells up
4. Verify that sub cells were configured without errors

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.