Stop testing with Postgres 12

[edgarrmondragon]

Postgres 12 is reaching EOL on November 14, 2024. Let's stop testing with it when the time comes.

The following files should be updated:

tap-postgres/README.md

Lines 16 to 18 in b550f86

	## Supported Python and PostgreSQL Versions
	This tap is tested with all actively supported [Python](https://devguide.python.org/versions/#supported-versions) and [PostgreSQL](https://www.postgresql.org/support/versioning/) versions. At the time of writing, this includes Python 3.9 through 3.13 and PostgreSQL 12 through 17.

tap-postgres/.github/workflows/test.yml

Lines 49 to 68 in b550f86

	matrix:
	python-version:
	- "3.13"
	- "3.12"
	- "3.11"
	- "3.10"
	- "3.9"
	postgres-version:
	- "17"
	include:
	- python-version: "3.13"
	postgres-version: "12"
	- python-version: "3.13"
	postgres-version: "13"
	- python-version: "3.13"
	postgres-version: "14"
	- python-version: "3.13"
	postgres-version: "15"
	- python-version: "3.13"
	postgres-version: "16"

https://www.postgresql.org/support/versioning/