Added post_migration to update plan titles

What does this MR do?

This MR updates the title of a couple of Plan records:

• Silver -> Premium (Formerly Silver)
• Gold -> Ultimate (Formerly Gold)

Up / Down migrations

etienne@dell ~/src/gdk/gitlab(300546-rename-persisted-plans ✔) rails db:migrate:up VERSION=20210201114748
== 20210201114748 RenamePlanTitles: migrating =================================
-- execute("UPDATE plans SET title='Premium (Formerly Silver)' WHERE name='silver'")
   -> 0.0006s
-- execute("UPDATE plans SET title='Ultimate (Formerly Gold)' WHERE name='gold'")
   -> 0.0279s
== 20210201114748 RenamePlanTitles: migrated (0.0286s) ========================

etienne@dell ~/src/gdk/gitlab(300546-rename-persisted-plans ✗) rails db:migrate:down VERSION=20210201114748
== 20210201114748 RenamePlanTitles: reverting =================================
-- execute("UPDATE plans SET title='Silver' WHERE name='silver'")
   -> 0.0005s
-- execute("UPDATE plans SET title='Gold' WHERE name='gold'")
   -> 0.0001s
== 20210201114748 RenamePlanTitles: reverted (0.0007s) ========================

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)