Change default branch name for GitLab.com instance

Problem to solve

As part of https://gitlab.com/groups/gitlab-org/-/epics/3600 GitLab has worked towards providing users flexibility in choosing a default branch name for their repositories both at the instance-level as well as the group-level.

The GitLab.com instance still uses master as its default branch name, something we'd like to change to main in order to align with Git's new default branch name as well as the community at large.

Proposal

Change default branch name in GitLab.com admin settings from master to main so that all new repos created from GitLab will use the new default.

Users will be able to change this for their particular group if they so desire (see https://docs.gitlab.com/ee/user/project/repository/branches/default.html#group-level-custom-initial-branch-name)

Timeline

Per #1356 (comment 572910815):

• Enable this setting on dev.gitlab.org on 2021-05-11
• Enable the setting on staging.gitlab.com on 2021-05-14, run QA and confirm that it is passing. Once confirmed, disable.
• Enable the setting on staging.gitlab.com on 2021-05-20, and leave enabled.
• Enable the setting on gitlab.com on 2021-05-24