Upgrade Postgres 11 -> 15

[ronardcaktus]

Adds PostgreSQL 15 support to align with the staging/production RDS PostgreSQL 11 -> 15 upgrade, including:

• Use postgresql-client-15 client in Dockerfile
• postgres-client-15 installs automatically in container (e.g. psql command exists)
• Use postgres:15 in docker-compose.yaml
• Use postgres:15 in .github/workflows/test.yml and tests pass
• Update to psycopg2 v2.9.9
• Update caktus.k8s-hosting-services PostgreSQL backup image to use 0.5.0-postgres15

[copelco]

Task linked: CU-86897mfb9 Philly Hip - Upgrade Postgres Version