fix: make user entity a data class

[ACMarcus]

• change from class to data class
• do not override abstract audit properties

[CLAassistant]

All committers have signed the CLA.

[mraible]

After watching this video, I learned it's a bad idea to use data for JPA classes.

[ACMarcus]

Might be, but as all other generated entities are generated as data class, why not also doing so for User entity?