Bust cache for GL.com billing page

What does this MR do?

Adds a cache key and feature flag subscription_plan_cache_key to bust the GL.com cache for the Billings page.

This is the implementation MR for https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/2537

FF Rollout issue URL

#299413 (closed)

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