Elasticsearch template needs updating

Elasticsearch 6 has removed support for the 'string' datatype [1].

Errors may be seen in the Monasca Log Persister logs such as:

19748d484f5d863e7896317-2020.08.20","_type":"log"},{"metaClass":{"metaClass":{"metaClass":{"action":"[\"index\", {:_id=>nil, :_index=>\"monasca-703c23c3219748d484f5d863e7896317-2020.08.20\", :_type=>\"log\", :routing=>nil}, #<LogStash::Event:0x2ef1331>]","response":{"index":{"_index":"monasca-703c23c3219748d484f5d863e7896317-2020.08.20","_type":"log","status":400,"error":{"type":"mapper_parsing_exception","reason":"Failed to parse mapping [log]: No handler for type [string] declared on field [@version]","caused_by":{"type":"mapper_parsing_exception","reason":"No handler for type [string] declared on field [@version]"}}}}}}}}]}}

[1] https://www.elastic.co/blog/strings-are-dead-long-live-strings