Which distro do we support ?

The installation instructions mention

> System requirements
>
> Python 3.8+
> PostgreSQL 9+
> Compiler toolchain and development files for Python and PostgreSQL
>
> On Debian-based distros, these can be installed with:
>
> sudo apt-get install python3-venv python3-dev postgresql libpq-dev build-essential
>

But as per https://packages.debian.org/buster/all/allpackages, all the packages listed above are already present in a debian buster installation.

We should explicitly mention which Debian we support ( and have some tests to prove that ). Supporting distros based on Debian is OK and dependencies for those should be mentioned separately.

A point to note: Our CI tests run on `ubuntu-latest` (we don't have a lot of choices there) and our Dockerfile uses `python:3.8` which on `buildpack-deps`