Drop the deprecated request-field fallbacks once clients stop sending them

[AmanGIT07]

Handler fallbacks that honor deprecated request shapes, blocked until the API formally drops those fields:

• internal/api/v1beta1connect/billing_usage.go — since (superseded by start_range)
• internal/api/v1beta1connect/billing_subscription.go — flat plan/immediate (superseded by plan_change)
• internal/api/v1beta1connect/permission.go — namespace/name body fields (superseded by key)
• internal/api/v1beta1connect/permission_check.go — split object fields (superseded by resource)
• cmd/serve.go — the FullyConsistent config flag (superseded by consistency)
• cmd/seed.go — seed data sends the deprecated permission body shape

Part of #1782.