Restoring from ODFE < 1.7 snapshot failing when upgrading to ODFE 1.7 and above

[jmazanec15]

In ODFE 1.7, we introduced the cosine similarity feature. To do this, we added an index setting called index.knn.space_type, which can be either l2 or cosinesimil. Restoring a snapshot from a previous ODFE version to a version >= 1.7 will cause cluster to go red, with an error that looks like this:

{"error":{"root_cause":[{"type":"snapshot_restore_exception","reason":"[<snap rep0>:<snap name>/<id>] cannot restore index [<index>] because it cannot be upgraded"}],"type":"snapshot_restore_exception","reason":"[<snap repo>:<snap name>/<id>] cannot restore index [<index>] because it cannot be upgraded","caused_by":{"type":"illegal_state_exception","reason":"unable to upgrade the mappings for the index [[<index>/<id>]]","caused_by":{"type":"mapper_parsing_exception","reason":"Failed to parse mapping [_doc]: setting
 index.knn.space_type has not been registered","caused_by":{"type":"illegal_argument_exception","reason":"setting index.knn.space_type has not been registered"}}}},"status":500}

This seems to fail in KNNVectorFieldMapper, here