WIP: Re-organize Plans metadata

What does this MR do?

This moves code around:

• Plans consts: it introduces metadata,
• Namespace: it delegates to gitlab_subscription as it has all data
• Removes names, rather relays on helper methods of plans, that indicate level
• Does not change any existing behaviour, but rather makes it more explicit

This tries to at least partially solve the split-brain that we have today:

• customers.gitlab.com provides it's own understanding of plans,
• plans do implement a separate set of behaviours,
• gitlab_subscriptions are to track changes made to plans
• the Plan also have two meanings in a codebase: License Plan (Ultimate/Starter/...) a GitLab.com Plan (Gold/Bronze/...)

Screenshots

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 #211481

Related to #34975 (closed)