Remove `group_export_ndjson` and `group_import_ndjson` feature flags

What does this MR do and why?

This MR removes the feature flags group_export_ndjson and group_import_ndjson which have been introduced in %13.0 and have been enabled by default since ~2 years. Additional changes in this MR:

• Removes references to group_export_ndjson and group_import_ndjson feature flags
• Removes the LegacyTreeSaver and the LegacyTreeRestorer (and related spec files)
• Adds an error message in case we try to import an old .json export (as opposed to .ndjson files)
• Updates documentation that import/export, starting in 15.7, will no longer support old .json exports

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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Related to #383682 (closed)