Remove the upgrade API

[clintongormley]

The upgrade API is no longer useful for adapting an old index for the next major version+1. Instead, the reindex API should be used instead.

We should remove the upgrade API.

[dadoonet]

I think we should do it when disabling source field will be removed: https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-source-field.html#_disabling_the_literal__source_literal_field

For now I don't believe we can remove this feature. No?

[rjernst]

@dadoonet We tried removing the ability to disable _source, but there was backlash. Why do you consider it to be related and blocking the removal of the upgrade api?

[dadoonet]

Reindex API reads _source right?

[clintongormley]

Yes it does, so reindex won't work if you disable source. That's par for the course. The upgrade API still won't help you to move to major version + 2.

[elasticmachine]

Pinging @elastic/es-core-infra