Update attr_encrypted config to use a more secure key and mode

This would mean implement https://gitlab.com/gitlab-org/gitlab-ce/issues/19073#note_12712458

It will involve a migration and updating all encrypted data. More research may also be done to avoid downtime (if possible) and find out which one would be the best mode to use. We'll also have to update the documentation in certain places and make sure the user has the right key length.

There are also differences between EE and CE - as EE has more encrypted fields.

mentioned in merge request !4963 (merged)

mentioned in issue #19073 (closed)

Added ~14106 label

mentioned in issue gitlab-com/infrastructure#177 (closed)

mentioned in issue #17975 (moved)

mentioned in issue #33254 (moved)

Closing in favour of https://gitlab.com/gitlab-org/gitlab-ce/issues/33254

closed