Include group `created_at` attribute in file exports for offline transfer
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## Summary
In direct transfer, groups are imported in two steps
1. The base portable is created in `BulkImports::Groups::Pipelines::GroupPipeline`. This pipeline makes GraphQL queries to the source destination, which won't be possible in offline transfer.
2. Attributes for the newly created portable are updated in `BulkImports::Groups::Pipelines::GroupAttributesPipeline` from `self` relation files.
`created_at` is fetched in `GroupPipeline` and not currently present in the `self` relation file, so it must be included in that file in order to import the source created date in offline transfer.
## Proposed implementation
Add `created_at` to `group/import_export.yml`.
### Effects on other importers
- File-based group import/export would now preserve the source group's created date. We probably want this, but it should be confirmed. We may need to communicate the change even though file-based group imports are deprecated
- Direct transfer would now preserve a group's `created_at`, if not already fixed in https://gitlab.com/gitlab-org/gitlab/-/issues/592140
issue