Set default image for mailroom container

For folks wanting to use Service Desk capability, they need to launch a mailroom container. By default this points to gitlab's own hosted image instead of registry1.dso.mil/ironbank/gitlab/gitlab/gitlab-mailroom:14.10.1. I'd like to have in the README listed the values to make that adjustment (and maybe default to the Registry1 version like the other images).

AC:

• Mailroom image defaulted to registry1.dso.mil/ironbank/gitlab/gitlab/gitlab-mailroom:15.0.1, pull secrets added as needed
• Renovate config updated (if needed) to track the mailroom image