Hibernate 6.6.x support

[jamesfredley]

Issue description

Issue to track Grails Data/GORM 6.6.x development for Grails 7/8.

grails-data-hibernate6 work has started and is a work in progress: https://github.com/apache/grails-core/tree/merge-hibernate6

Spring Boot 3.5.x, uses Jakarta persistence 3.1. Spring Boot 3.5.x spring-boot-dependencies points to hibernate 6.6.x: https://mvnrepository.com/artifact/org.springframework.boot/spring-boot-dependencies/3.5.4

Given grails-data-hibernate6 has ways to go, grails-data-hibernate5 will be around for a while and then there will be a gradual transition to hibernate6 before we stop hibernate5 development all together. hibernate5 will work with new Grails releases for a minimum of 15 months (May 2026) and if it works with Spring Boot 4.0.x with Spring Framework 7.0.x, then it will work with new Grails releases for some period of years since Spring sticks with an EE release (or two) across major versions. https://github.com/spring-projects/spring-framework/wiki/Spring-Framework-Versions#javajakarta-ee-versions

Grails hibernate6 support will likely start with hibernate 6.6 and support Grails 7.0.x initially.