Pin postgres version

Pin the version of Postgres to a known version so that renovate can see it and update it.

The version needs to be pinned in:

• https://gitlab.developers.cam.ac.uk/uis/devops/webapp-boilerplate/-/blob/master/template/.gitlab/webapp.gitlab-ci.yml?ref_type=heads#L26
• https://gitlab.developers.cam.ac.uk/uis/devops/webapp-boilerplate/-/blob/master/template/docker-compose.yml?ref_type=heads#L198

Suggestion is to pin at version 14 to meet minimum requirements for Django 5.2.