DB schema generated by a migration may look different in
rails 4 and 5 (because rails 5 may use different default values).
For this reason it's important to explicitly set for which rails
version a migration was written for.

See https://stackoverflow.com/questions/35929869/activerecordmigration-deprecation-warning-asks-for-rails-version-but-im-no/35930912#35930912

If milestone_id is NULL, we shouldn't need to update it to be NULL again. This
was causing us to touch almost all rows in the issues table for no good
reason.

Closes https://gitlab.com/gitlab-com/infrastructure/issues/3416

This adds various foreign key constraints, indexes, missing NOT NULL
constraints, and changes some column types from timestamp to
timestamptz.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/31811