Fix a test rake db:rollback: in .gitlab-ci.yml

What does this MR do?

• Fix feasible migration files to make it possible to rollback, such as

from

def change
     remove_column :projects, :main_language
end

to

def up
  remove_column :projects, :main_language
end

def down
  add_column :projects, :main_language, :string
end

• Set bundle exec rake db:rollback STEP=200 from bundle exec rake db:rollback STEP= 10000
  Because there are migration files to hard to fix, such as
• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/db/migrate/20160412173417_update_ci_commit.rb
• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/db/migrate/20160508221410_set_type_on_legacy_diff_notes.rb

And the first and difficult migration file (20160412173417) appear before 215 steps from the latest migration file(20170402231018).

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #29656 (closed)