Fix collaboration migration trigger helpers

[espadonne]

Summary

• update the SP21 collaboration migration to call the existing tg_set_updated_at() trigger helper
• verify a fresh local database migrates through 0107

Tests

• make lint-migrations
• env GOCACHE=/private/tmp/shithub-go-cache go test ./internal/migrationsfs ./internal/repos ./internal/web/handlers/repo
• env GOCACHE=/private/tmp/shithub-go-cache SHITHUB_DATABASE_URL=postgres://.../shithub_hotfix_0107_20260517 go run ./cmd/shithubd migrate up

Notes

• env GOCACHE=/private/tmp/shithub-go-cache make test still has unrelated local failures in internal/infra/storage setgid handling and webhook DNS resolution under the sandbox.