Suggests to use a JSON structured log instead

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/54102

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

Closes #43887.

The concurrency issue originates from inserts on
`user_interacted_projects` from the app while running the post-deploy
migration.

This change comes with a strategy to lock the table while removing
duplicates and creating the unique index (and similar for FK
constraints).

Also, we'll have a non-unique index until the post-deploy migration is
finished to speed up queries during that time.

Closes #44205.

This is to avoid a mix-up with the existing concept of 'user
contributions'. See `User#contributed_projects` or
`Event#contributions`.

MySQL does not return the number of inserted rows (or at least not
expose them through cmd_tuples). Let's do one-shot INSERT instead.

Trying to be a good citizen here, we spread the load building that
table. Maybe not required, but at least good practice and no harm.