Grails 7: document how to add hibernate ehcache dependency

[jamesfredley]

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.

The gorm documentation (under Getting Started) recommends ehcache be included.

Yes, exactly! The documentation does not say that hibernate-ehcache is provided transitively. It says to include it explicitly.

We need to be sure to update the gorm docs for these workarounds so people understand this is only until we have hibernate 6.

Yes, it looks very outdated and needs to be updated.