MNT Use php 7.4 for prefer-lowest

[emteknetnz]

Issue silverstripe/gha-ci#37

Fix https://github.com/silverstripe/recipe-kitchen-sink/runs/7512733988?check_suite_focus=true
Problem 1 - Root composer.json requires php ^7.4 || ^8.0 but your php version (7.3; overridden via config.platform, actual: 7.3.33) does not satisfy that requirement.

[GuySartorelli]

Are we going to have to update these every release that changes PHP versions?
Is there a way to add these jobs without having to define the PHP versions in here?

[emteknetnz]

Nope. It's really not a big deal though, probably only one more release on the 4.x line and I'm not sure if we'd drop 7.4 at that point

[GuySartorelli]

Does that mean for the 5.x line we'll be doing something different? My main worry is that we'll forget to update this since we don't change PHP versions very often.

[emteknetnz]

Build will break because the version of PHP is too low for what composer.json accepts so we'll know. That's how I knew for this one.

[GuySartorelli]

Yeah I guess.... it's not great but it works for now so I'll merge it and we can look at reworking this if it becomes a burden down the line.