Remove the fully-enabled feature flag and simplify the billing metadata
API to always use the vNext endpoints. The legacy API path is removed
since the server will no longer serve it.
- Remove FeatureFlag.PM25379_UseNewOrganizationMetadataStructure enum and default
- Delete legacy getOrganizationBillingMetadata() API method (old /billing/metadata path)
- Rename vNext methods to remove VNext suffix
- Simplify OrganizationMetadataService to always use cached vNext path
- Remove ConfigService dependency from OrganizationMetadataService
- Update tests to remove feature flag branching