Grails 7: document how to add hibernate ehcache dependency

[jamesfredley]

Issue description

This was historically provided by the grails hibernate5 plugin, but that plugin does not directly use the dependency, so it was removed. We need to document the process to add ehcache to end Grails 7 applications, which is slightly complicated by the jakarta vs javax versions of hibernate 5.6.x.

https://github.com/grails/grails-core/blob/7.0.x/UPGRADE7.md