Require PHP 7.4

Problem/Motivation

The module is using typed class properties, which were introduced first in PHP 7.4. The module therefore works only with PHP >= 7.4 and trying to use it with PHP 7.3 throws errors.

Steps to reproduce

Run the module with PHP 7.3 and e.g. clear caches.

Proposed resolution

Require PHP >= 7.4 in composer.json.

Remaining tasks

User interface changes

API changes

Data model changes