[PHP8.1] use `readonly` properties

[jkowalleck]

currently readonly properties are private, annotated as @readonly and available via a getter.

since php8.1 native support for readonly properties exists.

proposal

• remove the getter
• make the property public
• make the property readonly - via native readonly propertes #208

attention

• there are issues when it comes to modifications in __clone() - -therefore, not fully rolled out
• there are extended works needed to make the test cases aware ...
• failed try: [WIP] use readonly properties #207

[jkowalleck]

there are a couple of readonly properties, now.
some are private, but there is still a getter for them. why? this can be optimized.