Update background migration to use main instead of master

https://gitlab.com/gitlab-org/gitaly/-/merge_requests/3537 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`, but the default branch was `main`.

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

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/336538

lib/gitlab/background_migration/backfill_snippet_repositories.rb

@@ -105,7 +105,7 @@ module Gitlab
       end
 
       def commit_attrs
-        @commit_attrs ||= { branch_name: 'master', message: 'Initial commit' }
+        @commit_attrs ||= { branch_name: 'main', message: 'Initial commit' }
       end
 
       def create_commit(snippet)

spec/lib/gitlab/background_migration/backfill_snippet_repositories_spec.rb

@@ -304,7 +304,7 @@ RSpec.describe Gitlab::BackgroundMigration::BackfillSnippetRepositories, :migrat
   end
 
   def blob_at(snippet, path)
-    raw_repository(snippet).blob_at('master', path)
+    raw_repository(snippet).blob_at('main', path)
   end
 
   def repository_exists?(snippet)