pg 15 and 16 packer/ansible/ghactions

[samrose]

What kind of change does this PR introduce?

This PR is a child PR of #1266

It includes the changes to packer/ansible/gh actions to allow for a matrix build of pg versions on release.

Significant changes

This PR moves the source of truth for data about postgres major and semver back to ansible/vars.yml, which are then read at time of release in the ami-release gh action. The common-vars.nix.yml file is no longer under source control, but instead is generated duing the ci run for ami release or testinfra runs. The version is now controlled in this allows us to avoid making too many changes to our release and test tooling for AMI)

To bump the versions of postgres it is proposed to update these values that are now in anisble/vars.yml

These versions cover both release and testinfra

postgres_major:
  - "15"
  - "16"

# Full version strings for each major version
postgres_release:
  postgres15: "15.8.1.003-staging-5"
  postgres16: "16.3.1.000-staging-5"