Update docs in /gitlab/doc/api to replace "master" with "main"

doc/api/branches.md

doc/api/commits.md

doc/api/events.md

doc/api/group_badges.md

doc/api/group_protected_branches.md

doc/api/group_wikis.md

doc/api/groups.md

doc/api/issues.md

doc/api/merge_request_approvals.md

doc/api/merge_requests.md

doc/api/packages.md

doc/api/project_badges.md

doc/api/project_vulnerabilities.md

doc/api/projects.md

doc/api/repositories.md

doc/api/repository_files.md

doc/api/repository_submodules.md

doc/api/runners.md

doc/api/search.md

doc/api/snippets.md