In concert with the wider Git community, GitLab committed to changing
the default branch from "master" to "main" in 14.0. This change has
already happened on GitLab.com; this change enables it for
self-managed instances as well.

GitLab has support for a custom default branch name which can be used
to override this change if defaulting to "main" is inconvenient.

Unless overridden, this affects all new projects on all self-managed
GitLab instances. Existing projects will not have their default branch
changed.

Changelog: added