Postgres errors builds on Debian prior to Bullseye

[reynoldsalec]

In the image builds or if you try building an app that has build steps involving apt update on versions prior to PHP 7.4, you'll get the following error:

E: The repository 'http://apt.postgresql.org/pub/repos/apt buster-pgdg Release' does not have a Release file.

See example in failed test run: https://github.com/lando/php/actions/runs/11244325076/job/31262133348?pr=129#step:7:204

Looks like that repository no longer has a release file for buster and older Debian images; I had to upgrade to php:7.4 to get one of my apps working.

Sorta related to #119, unfortunately older versions of the PHP official image don't have builds on newer versions of Debian, so we'll have to work around this another way.

[reynoldsalec]

Would also resolve lando/drupal#42 in a more permanent fashion.