Unsupport 4.x - 5.x library has been stable long enough

Problem/Motivation

https://photoswipe.com/ 5.x is stable long enough now, so I think we should unsupport the 4.x branch of the Drupal module.
Who wants to use it, can still do, but we shouldn't support it actively anymore.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes