`safe_find_or_create_by!` creates a savepoint if already in transaction,
which is usually the case for migrations. This updates the helper to
use 'find_or_create_by!` instead in order to avoid subtransactions.

Also changes the helper to update index definition if the record already
exists and there is new definition, something we state in the method
comment but were not actually doing.

https://gitlab.com/gitlab-org/gitlab/-/issues/339667