scheduler: tasks archival: Test run on test elasticsearch cluster

As the title says, experiment on @ftigeot's elasticsearch cluster to try and archive tasks. And report back new found errors here then fix the code accordingly.

This is currently running and raised unforeseen errors (during the development phase).

Note:

• The tasks are the real ones (db: softwareheritage-scheduler)
• The tryout run does not delete the archived/indexed tasks.

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Migrated from T1023 (view on Phabricator)