Just remove the version constraint:
# as of today it gives you 5.1.2
composer create-project --prefer-dist cakephp/app cms
And if you really want to put a version constraint in
composer create-project --prefer-dist cakephp/app=^5.1.2 cms
And if you want to upgrade to the latest cakephp (5.2.5)
cd cms
composer require cakephp/cakephp:^5.2