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If you're more used to GitLab than Drupal.org, you might spend a while hunting through settings there to figure out how to change the default branch - or concluding that you simply can't. I'm posting this q&a in case it saves someone else the hassle of either time waster.
Fwiw, if you are looking for the non-Drupal.org answer, see https://docs.gitlab.com/ee/user/project/repository/branches/#default-branch or https://stackoverflow.com/questions/30987216/change-default-branch-in-gitlab
Turns out it's a setting on your Drupal.org version of the module page. Visit edit > default branch on the page-specific menu of your module.
At the time of creating this question, the documentation page from 2012 (last updated 2015) is still close enough to be helpful.
