cakephp/app is a different repository from
cakephp/cakephp. So it follows it's own versioning.
cakephp/app is a template/skeleton that you can use to create your app. While cakephp/cakephp is the main core.
Check below for the cakephp/app:
I guess you could use the version
3.1.2 of cakephp/app that allows you to have PHP >= 5.4