Upgrade to PHP 8.3 features

Problem/Motivation

The code originates from a PHP 7.1 minimal base.

We can adopt PHP 8.1 features based on our currently supported PHP version.

Steps to reproduce

Proposed resolution

Adopt:

• Property promotion
• Typed properties
• Union Types (if relevant)
• Any other features that may be observed as useful while auditing code

Remaining tasks

User interface changes

API changes

Data model changes