Dynamically rename plan title after EoA rollout date

What does this MR do?

Following the Bronze/Starter plan end-of-availability (EoA) rollout, some plans in gitlab.com still mentions Gold or Silver even if these plans were purchased after that rollout.

This MR offers a short-term solution (until we properly synchronize names across gitlab.com and customers.gitlab.com - see details in this comment) : it dynamically renames plan title after EoA rollout date, which was January 26th 2021

Screenshots (strongly suggested)

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)