Remove the deprecated API usage acknowledged in the staticcheck rollout

[AmanGIT07]

PR #1781 annotated every remaining use of a deprecated API with //nolint:staticcheck. Each annotation is deliberate, but all of them are debt. This issue tracks removing them. The sites, grouped by what unblocks each:

Actionable now

• New Relic go-agent v2 → v3 — pkg/db/db.go and internal/store/spicedb/relation_repository.go import the deprecated v2 package. Migrating means the v3 import path and the reworked transaction/segment APIs at every telemetry call site.
• e2e billing tests — test/e2e/regression/billing_test.go uses the deprecated UpdateBillingAccountLimits RPC as a setup tool. UpdateBillingAccountDetails sets the same credit_min (note it also writes due_in_days).

Blocked on the API dropping the deprecated request fields

Handler fallbacks that must stay while old clients send the old shapes:

• internal/api/v1beta1connect/billing_usage.go — since (superseded by start_range)
• internal/api/v1beta1connect/billing_subscription.go — flat plan/immediate (superseded by plan_change)
• internal/api/v1beta1connect/permission.go — namespace/name body fields (superseded by key)
• internal/api/v1beta1connect/permission_check.go — split object fields (superseded by resource)
• cmd/serve.go — the FullyConsistent config flag (superseded by consistency)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

Blocked on a lossless replacement for the deprecated response fields

internal/reconcile/permission_reconciler.go, cmd/permission.go, and the assertions in test/e2e/regression/service_registration_test.go read the deprecated namespace/name response fields. The replacement key round-trips lossily today: a namespace without a slash gains a /default suffix through SplitNamespaceResource, and names containing dots fail to split. These readers can migrate once key (or another field) carries the exact values.