Change composer.json to allow PHP8

[hacheraw]

I'm using CakePHP 4.3 running on PHP8.1 and get no errors/deprecations and everything is working as expected.
The only deprecation I get is using Cake\Console\Shell instead of Commands (they will be removed in CakePHP 5.0)

Could you please update composer.json to something like "php": ">=7.2 <9", so we don't have to use composer update --ignore-platform-reqs

I could to a PR if >=7.2 <9 is good for you.

Thanks!!!