Bulk propagating integrations sets the created_at and updated_at timestamps

We currently set the created_at and updated_at timestamps of project-level integrations to the instance-level timestamps when we propagate changes to them. This also affects the data field tables. See #355610 (comment 929543539).

Our database schema for integrations also allows null timestamps.

Proposal

We should:

• When bulk creating, set created_at and updated_at to current time.
• When bulk updating, set only the updated_at to current time (and not change the existing created_at value).
• Migrate a not null constraint for created_at and updated_at columns of integrations.