docs: Add Grails 7 gotcha about jar artifact naming (#13825)

UPGRADE7.md

@@ -6,6 +6,8 @@ Experienced while upgrading modules for Grails 7
   - https://github.com/grails/gorm-hibernate5/pull/910/commits/c8de45df204966ccc228b46b94beeb2142ae0f59 
 - [GROOVY-10621](https://issues.apache.org/jira/browse/GROOVY-10621) 
   - Primitive booleans will no longer generate the form of isProperty & getProperty.  They will only generate isProperty()
+- Jar artifacts produced by Grails Plugins will no longer have the suffix `-plain`
+  - https://github.com/grails/grails-gradle-plugin/pull/347
 - [GROOVY-5169](https://issues.apache.org/jira/browse/GROOVY-5169)  [GROOVY-10449](https://issues.apache.org/jira/browse/GROOVY-10449)
   - Fields with a public modifier were not returned with MetaClassImpl#getProperties() in groovy 3, but are now.