Steps to reproduce the issue

Register own handler for onAfterRenderModules event and try to modify the $buffer passed as first argument.

Expected result

$buffer is passed by reference in event trigger and should be allowed to be modified by plugins.

Actual result

No modifications are applied because we can't access Joomla\Event\Event argument by reference.

Additional comments

Either Joomla\Event\Event should allow argument modifications by reference, or onAfterRenderModules event should use object argument with buffer property returned by ModulesRenderer::render() (but this breaks BC).