Encrypt migration credentials at rest (#15895)

Encrypt migration credentials at rest (#15895)

* encrypt migration credentials in task persistence

Not sure this is the best approach, we could encrypt the entire
`PayloadContent` instead. Also instead of clearing individual fields in
payload content, we could just delete the task once it has
(successfully) finished..?

* remove credentials of past migrations

* only run DB migration for completed tasks

* fix binding

* add omitempty

* never serialize unencrypted credentials

* fix import order

Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>