Drop dependency on PHP 7.4 to support PHP 7.3

Problem/Motivation

I'm currently developing a project. The hosting provider that is used has some restrictions, and the highest PHP-version that is available is PHP 7.3 (Yes, I know it's no longer supported).

I'm testing this module on PHP 7.3 and since Drupal 9 still supports PHP 7.3 it perhaps makes sense to drop all PHP 7.4 related code and to support PHP 7.3 again. Once support for Drupal 9 is dropped, PHP 7.4 specific syntax can be re-added.

Since the constraint on PHP 7.4 or higher was added pretty late in the lifecycle of this module, there are 3 small changes necessary to support PHP 7.3.

WDYT?

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes