Use loose foreign key nullify for `User.builds` and `User.pipelines`

Related to #339813 (closed).

Once we have the loose foreign keys implementation in !69165 (merged) we can use it to replace the dependent: :nullify in:

User.has_many :builds, dependent: :nullify, class_name: 'Ci::Build' 
User.has_many :pipelines, dependent: :nullify, class_name: 'Ci::Pipeline'

These are both causing problems with transactions that span writes to different databases.

See more discussion at #339813 (comment 670693502)