docs: the PostgreSQL minimum says 14, and CI tests 16 only #898

The README gives PostgreSQL 14 as the minimum version (README.md, Requirements), and #897 keeps 14 in the stable tier. CI runs PostgreSQL 16 only (.github/workflows/integration.yml: the postgres:16 and pgvector/pgvector:pg16 services). PostgreSQL 14 reaches its upstream end of life on 2026-11-12.

These files also state the minimum or install an older version, at main 3033e09:

• CONTRIBUTING.md:10, AGENTS.md:13 and :102, Dockerfile:68, docker/README.md:12, docker/dockerhub-overview.md:9, and .github/workflows/dockerhub-description.yml:161 and :284 state 14.
• docs/manuals/04-quickstart-setup-guide.md:97 installs postgresql14, docs/manuals/08-install-linux.md installs postgresql15, and docs/local-postgres-setup.md uses 15.17.

Recommendation: state 16 as the minimum in every file above and in the README. PostgreSQL 14 reaches end of life before 1.0, and CI tests no other version. The other option is a CI job on PostgreSQL 14.