[Task Manager] remove [DOC_TYPE] from index template body

[tsullivan]

Summary

Starting Kibana with Task Manager enabled broke recently due to incompatibility with ES in the way it was trying to put the index template. The ES change seems to be: elastic/elasticsearch#38270

This keeps the type name in the mapping to make maintaining 6.x with backports easier.

This fixes an error when starting Kibana:

server    log   [16:32:11.715] [info][task_manager] Installing .kibana_task_manager index template version: 8000099.
server    log   [16:32:11.727] [warning][task_manager] [illegal_argument_exception] The mapping definition cannot be nested under a type [_doc] unless include_type_name is set to true.

[jpountz]

Ideally on 7.0+ we would instead remove [DOC_TYPE]: { a couple lines below. This fix "works" but is deprecated since we are moving towards removing the notion of types.
This is the right approach on 6.7 however.

[tsullivan]

Ideally on 7.0+ we would instead remove [DOC_TYPE]: { a couple lines below. This fix "works" but is deprecated since we are moving towards removing the notion of types.
This is the right approach on 6.7 however.

That's fine. We can do a different fix for 7.0+ and 6.7. Updates coming.
cc @tylersmalley

[tsullivan]

Just the 1st commit will be backported to 6.7

[elasticmachine]

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[elasticmachine]

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[elasticmachine]

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[tsullivan]

Test failures are known/unrelated.

Error: Unable to download elasticsearch snapshot: Not Found

Kibana test server isn't able to download an 8.0.0 snapshot for Elasticsearch, as that version hasn't been published yet

[elasticmachine]

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