deps: upgrade to postgres 15 bookworm

steps to upgrade db image:

1. backup db via `npm run backupdb` in the server container
2. copy the generated pom-(date).sql out from the server container into
   the host, e.g., `docker compose cp server:/code/server/pom-db.sql .`
3. stop db container
3. upgrade to new postgres image in docker-compose.yml
4. backup postgres data directory e.g., `mv docker/data{,.backup}`
5. regenerate `docker-compose.yml` via `make docker-compose.yml` or
   `make deploy`
6. `docker compose up -d`
7. copy pom-db.sql into server container and run
   `npm run dangerously-loaddb`

resolves virtualcommons/planning#48

base.yml

@@ -33,7 +33,7 @@ services:
       interval: 10s
       timeout: 5s
       retries: 5
-    image: postgres:12
+    image: postgres:15-bookworm
     secrets:
       - pom_db_password
     restart: always