The Admin V3 styles are now the primary styles for the application. This change promotes the `admin_v3` directory to `admin` and archives the old styles.
Changes:
- Renamed `app/webpacker/css/admin_v3` to `app/webpacker/css/admin`.
- Renamed the previous `app/webpacker/css/admin` to `app/webpacker/css/admin_legacy`.
- Moved all files referenced by V3 styles from the legacy directory to the new `admin` directory.
- Updated imports in `all.scss` to distinct local files instead of relative paths.
- Cleaned up `admin_legacy` by removing files that are duplicates (by name or content) of the new admin styles.
- Updated `admin-style-v3` pack to point to the new location.
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