Bump minimum required PostgreSQL version to 15

[jeremystretch]

Proposed Changes

Raise the minimum required version of PostgreSQL from 14 to 15.

Justification

PostgreSQL 14 will be end-of-life in November 2026. Django 6.1 will require PostgreSQL 15+ when it is released in August 2026.

Impact

Any existing NetBox deployments running PostgreSQL 14 will need to be upgraded to 15 or later before upgrading to the NetBox release in which this change is made.