Postgres 14 nearing EOL — upgrade to a newer LTS version

### Summary

`sentry/self-hosted` currently deploys `postgres:14.23-bookworm`. PostgreSQL 14 reaches end-of-life in September 2026 (~2 months from now). The project should adopt a newer LTS version before support ends.

### Findings

- `docker-compose.yml:166` pins the image to `postgres:14.23-bookworm` and explicitly notes it is kept in sync with the Sentry dev environment.
- `install/upgrade-postgres.sh` only covers the 9.6 → 14 migration path; a newer major version would require a new migration plan.
- Dependabot PR #4413 previously attempted to bump to `18.4-bookworm` but was closed without merging.

### Related

- #4413

Start with docker-compose.yml:166 to inspect the current PostgreSQL image pin and its synchronization note, then read install/upgrade-postgres.sh to understand the existing 9.6 → 14 migration path. Review closed PR #4413 for the attempted version bump. Done means the deployment uses a supported newer PostgreSQL version and the upgrade path covers the required migration.

Written by the indexing model from the issue text.