Remove the deprecated API usage acknowledged in the staticcheck rollout

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• internal/api/v1beta1connect/billing_usage.go — since (superseded by start_range)
• internal/api/v1beta1connect/billing_subscription.go — flat plan/immediate (superseded by plan_change)
• internal/api/v1beta1connect/permission.go — namespace/name body fields (superseded by key)
• internal/api/v1beta1connect/permission_check.go — split object fields (superseded by resource)
• cmd/serve.go — the FullyConsistent config flag (superseded by consistency)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

Start with PR #1781 and the actionable files: pkg/db/db.go, internal/store/spicedb/relation_repository.go, and test/e2e/regression/billing_test.go. Trace the deprecated telemetry calls and billing setup usage, then check the listed blocked handlers and readers to understand their dependencies. Done means the actionable deprecated usages are removed and the remaining annotations have a clear API dependency preventing removal.