Upgrade and pin unified container's postgres version

[alexdriedger]

• Upgrade unified container's internal postgres version from 15 to 17 (docker compose is already on 17)

• Pin the postgres version in the unified dockerfile

• NOTE: If all these following conditions are true for you, then you will need to run pg_upgrade to upgrade the internal postgres instance (see https://www.postgresql.org/docs/current/upgrading.html):

1. You use are using the pontoon-unified container
2. You are NOT using an external postgres db (we recommend using an external postgres db for production deployments)
3. You want to keep the existing data for your pontoon instance