Support for ES 8.x

I'm getting the below error while trying to insert records to ES 8.x using latest aiven connector

**_Encountered an illegal document error when executing batch 267 of 10 records. Ignoring and will not index record. Error was {"root_cause":[{"type":"illegal_argument_exception","reason":"Action/metadata line [1] contains an unknown parameter [_type]"}],"type":"illegal_argument_exception","reason":"Action/metadata line [1] contains an unknown parameter [_type]"} (io.aiven.connect.elasticsearch.bulk.BulkProcessor:414)_**

Connector config has this set
```
"schema.ignore": "true",
"type.name": "_doc",
"value.converter.schemas.enable": "false",
"value.converter": "org.apache.kafka.connect.json.JsonConverter"
```