Fix rfc3339 datetime migration

[Alenar]

Content

This PR fix the migration that migrate dates in the aggregator database to the rfc 3339 format.
The created_at & artifact columns of the signed_entity table were swapped, leading to an error when querying any artifacts.

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Issue(s)

Relates to #946, Fix a bug introduced in #994

[Alenar]

The following script allow to visualize all dates in the database, usefull to check that they are all on the migrated to rfc3339:

select * from (select 'certificate' as 'table', 'initiated_at' as 'column', initiated_at as 'value' from certificate limit 2)
union select * from (select 'certificate', 'sealed_at', sealed_at from certificate limit 2)
union select * from (select 'open_message', 'created_at', created_at from open_message limit 1)
union select * from (select 'signer_registration', 'created_at', created_at from signer_registration limit 1)
union select * from (select 'signed_entity', 'created_at', created_at from signed_entity limit 1)
union select * from (select 'signer', 'created_at', created_at  from signer limit 1)
union select * from (select 'signer', 'updated_at', updated_at from signer limit 1)
union select * from (select 'single_signature', 'created_at', created_at from single_signature limit 1)
union select * from (select 'stake_pool', 'created_at', created_at from stake_pool limit 1)
union select * from (select 'db_version', 'updated_at', updated_at from db_version limit 1)

[jpraynaud]

LGTM 👍