Drop support for PostgreSQL 13

### Proposed Changes

Require PostgreSQL 14 or later beginning with NetBox v4.3.

### Justification

Django 5.2 [no longer supports PostgreSQL 13](https://docs.djangoproject.com/en/dev/releases/5.2/#dropped-support-for-postgresql-13). Upstream support for PostgreSQL 13 ends in November 2025.

### Impact

Users running PostgreSQL 13 will need to upgrade.