Remove `group_export_ndjson` and `group_import_ndjson` feature flags

The feature flags group_export_ndjson and group_import_ndjson have been introduced in %13.0 and are enabled by default since ~2 years. Let's remove the code reference and check if there is related legacy code that can be removed along (such as Gitlab::ImportExport::Group::LegacyTreeSaver)

TODOs

• Remove references to group_export_ndjson and group_import_ndjson feature flags
• Remove the LegacyTreeSaver and the LegacyTreeRestorer
• Add an error message in case we try to import an old .json export (as opposed to .ndjson files)
• Update documentation that import/export, starting in 15.7, will no longer support old .json exports (We'd have to double check which is the last version supported)