[4.4] Events backward compatibility handling

[Fedik]

Summary of Changes

Same as #41345 but for 4.4.
Fixing possible b/c break for an events.
I have extracted it from #41226 in to separated PR.

Testing Instructions

Apply patch.
Edit plugins/system/sef/src/Extension/Sef.php, add return true; at top of onAfterDispatch() function.
Run in index.php of a template:

$event = new class('onAfterDispatch') extends Joomla\CMS\Event\AbstractImmutableEvent{};
$app->getDispatcher()->dispatch('onAfterDispatch', $event);

Actual result BEFORE applying this Pull Request

An error, cannot set result for Imutable event

Expected result AFTER applying this Pull Request

No error

Link to documentations

Please select:

• Documentation link for docs.joomla.org:
• No documentation changes for docs.joomla.org needed
• Pull Request link for manual.joomla.org: Use numeric access to named event arguments Manual#169
• No documentation changes for manual.joomla.org needed

[HLeithner]

I have tested this item ✅ successfully on d09f3e1

Works with patchtester which broke before

---

_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/41357.}

[laoneo]

Merging this now. Thanks @Fedik, can you write a manual entry for the deprecation notice?

[Fedik]

There it is joomla/Manual#169