Magento 2 reindexing output message

Hello,

We have elastic Search for magento 2.2.2 and on reindexing using SSH we get this output:

Catalog Search indexer process unknown error:
{"error":{"root_cause":[{"type":"mapper_parsing_exception","reason":"No handler for type [keyword] declared on field [clasp_type]"}],"type":"mapper_parsing_exce ption","reason":"No handler for type [keyword] declared on field [clasp_type]"}, "status":400}

Elastic Search 2.4.6

keyword type does not exist in elasticsearch 2.x.
You need to upgrade.

What version would be the best to upgrade?

Best regards,
scoops

7.2.0 is the latest.