Migration to add new plans

What does this MR do?

This MR adds two new records into the plans table for the new Premium and Ultimate plans.

As mentioned in this comment, it's part of a long-term solution to replace Gold, Silver Plan instances currently used in gitlab.com. This addition of records in the plans is the first step.

(Note: while working on this solution, a short-term solution will be applied)

Up and Down migrate

etienne@dell ~/src/gdk/gitlab(300546-add-new-plans ✗) rails db:migrate:up VERSION=20210205104425                                                                                                                     
== 20210205104425 AddNewPostEoaPlans: migrating ===============================
-- execute("INSERT INTO plans (name, title, created_at, updated_at) VALUES ('premium', 'Premium (Formerly Silver)', CURRENT_TIMESTAMP, CURRENT_TIMESTAMP)")
   -> 0.0284s                                                                                             
-- execute("INSERT INTO plans (name, title, created_at, updated_at) VALUES ('ultimate', 'Ultimate (Formerly Gold)', CURRENT_TIMESTAMP, CURRENT_TIMESTAMP)")
   -> 0.0002s                                
== 20210205104425 AddNewPostEoaPlans: migrated (0.0288s) ======================
                                                                                                          
etienne@dell ~/src/gdk/gitlab(300546-add-new-plans ✗) rails db:migrate:down VERSION=20210205104425
== 20210205104425 AddNewPostEoaPlans: reverting ===============================
-- execute("DELETE FROM plans WHERE name IN ('premium', 'ultimate')")
   -> 0.0011s                                                                                             
== 20210205104425 AddNewPostEoaPlans: reverted (0.0012s) ======================

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #300546 (closed)