Improve instructions for PHP74

Problem/Motivation

PHP7.4 is available from NixPkgs, but only if you specify an older version of Nixpkgs -- the current nixos-unstable branch has dropped support for PHP older than 8.1.

Steps to reproduce

Set up a project, create a .env file, and specify "php74" as the php version.

Proposed resolution

Provide an older nixpkgs input for some older php versions.

Remaining tasks

PHP74 does not provide packages for codemirror or phpunit, so these need to get skipped in the PHP services flake part.

User interface changes

API changes

Data model changes