Update background migration to use main instead of master [RUN AS-IF-FOSS]

Stan Hurequested to merge
sh-fix-issue-336538 into master

gitaly!3537 (merged) updated the default branch to be master instead of main. This broke backfill_snippet_repositories_spec.rb because it expected to create commits in master (https://gitlab.com/gitlab-org/gitlab/blob/e1acca6ead3c7a1f5ea2b5aff37e2aa552b438fd/lib/gitlab/background_migration/backfill_snippet_repositories.rb#L108), but the default branch was main.

Since main is now the default, we should target the initial commit for that branch.

Closes #336538 (closed)

Edited by Stan Hu

Merge request reports