Allow both CMS 4 and CMS 5 installs

[NightJar]

The upgrade from CMS 4 to CMS 5 support involved a single number change in composer.json. I propose this should have been ^4 || ^5 instead.

There is no dependency difference between the two branches (no API change was needed for CMS 5 support), and fixes from the main branch are having to be "backported" (#58 & #59) to the CMS 4 branch, where rebase "merging" has occurred - diverging the commit history. This likely makes the normal "merge-up" routine more difficult.

Simplifying maintainer support by changing the framework requirement (and probably removing the CMS requirement too - no code relies on it) would make things easier for everyone until such time that an actual codebase deviation is required for multiple version support.

[NightJar]

CMS 4 is end of life in just a few months.
It's too late for this now. The 2 branch can be end of life too in time with the CMS.