Docs: Update GitHub master to main

Problem to solve

GitHub has changed the name of their default branch from master to main.

We reference the GitHub branch name in the docs, for example: https://docs.gitlab.com/ee/user/project/repository/reducing_the_repo_size_using_git.html#purge-files-from-repository-history

This issue is to update our docs to reflect GitHub's new name.

Further details

Proposal

Who can address the issue

Other links/references

Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/221164