No need for scripts/db_tasks script for db:migrate

What does this MR do and why?

Fixes the following jobs to just run rake db:migrate. This provides a more realistic test as rake db:migrate will migrate both main , and ci database - there is no need to artifically restrict to rake db:migrate:main (which is what script/db_tasks does)

• db:rollback
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/3828048452
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/3828048453
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/3828048448
• db:check-schema
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/3828048456
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/3828048457
  • https://gitlab.com/gitlab-org/gitlab/-/jobs/3828048450

Related issue: #386436

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