Is there a sub-version for CI_VERSION, if not why not :)

Basically, I use the latest develop branch in codeigniter4/CodeIgniter4 repository.
It is the latest development code. If a bug can't be reproduced on it, it is probably fixed already.

When you report a bug, if you use codeigniter4/CodeIgniter4 repository,
the commit hash is most accurate version. If you show it, we can know
the date and time.

`4.1.5-2021-11-28` is a bit confusing. Because there is only one 4.1.5 code base.
If one commit is added to it, it is not 4.1.5.
`develop 2021-11-28` or just `develop` is better.

ci-phpunit-test | CodeIgniter Testing Guide | CodeIgniter 3 to 4 Upgrade Helper