Update CKEditor dependency to use CKEditor 5 instead of deprecated CKEditor 4

Problem/Motivation

CKEditor 4 is being deprecated, therefore we need to upgrade Anu LMS to support / work with CKEditor 5 instead of CKEditor 4.

Remaining tasks

- Update configurations of text editors to use CKEditor 5 configs instead of CKEditor 4 configs
- Update module's dependency
- Update hook for automated migration?

User interface changes

Visually CKEditor 5 looks different (subjectively - much better!) than CKEditor 4.