Migrate to php-fpm config from Services Flake

Problem/Motivation

After a routine flake update, the environment does not build, due to a namespace conflict with services.php-fpm. This is because PHP-FPM has been added to the upstream Services Flake.

Steps to reproduce

Run nix flake update, and attempt to use nix develop.

Proposed resolution

For the short term, I have rolled back the services-flake source in the flake.lock -- if you use the flake.lock from this project now, it should continue to work.

Refactor custom php-fpm service to use the upstream from Services Flake.

Remaining tasks

User interface changes

API changes

Data model changes