Should address #15285

• I'm removing a hard coded GrailsVersion (except on the bom) since we can assume either the dependency management plugin or the gradle bom has been applied.
• adjusting old coordinates to the newer coordinates
• consistent // ... to indicate that other Grails dependencies can be included
• prefer rxjava3 over rxjava1 since rxjava1 is the original version
• add rxjava3 to the events documentation