Prepare `...Group::TreeRestorer` to ndjson

#212327 (closed)

What does this MR do?

Renaming Gitlab::ImportExport::Group::TreeRestorer to Gitlab::ImportExport::Group::LegacyTreeRestorer is part of the effort to enable ndjson on the Group Import/Export feature. With ndjson enabled all the structure of restoring a group-tree is different, so to make the maintenance easier the "non-ndjson" structure is being renamed.

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Originally all the effort to enable ndjson on for Group Import/Export feature was done in one MR, which endup being too big. @ayufan then suggested to split that work on smaller MRs to make the work easier to review.

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Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team