Drop support for PostgreSQL 11

[jeremystretch]

Proposed Changes

Increase the minimum supported PostgreSQL version from 11 to 12.

Justification

Django 4.2 requires PostgreSQL 12 or later. The last release of PostgreSQL 11 will be in November 2023.

Impact

Existing NetBox deployments will need to upgrade to PostgreSQL 12 or later before upgrading to NetBox v3.6.