[4.4] Backward compatibility handling for plugins that setting the result directly

[Fedik]

Summary of Changes

Addittionaly to #41357 the plugin may set the event result directly $event['result'] = $result.
Example in core Debug plugin

joomla-cms/plugins/system/debug/src/Extension/Debug.php

Lines 387 to 391 in c4ee583

	$result = $event['result'] ?: [];
	$handler = new OpenHandler($this->debugBar);
	$result[] = $handler->handle($this->getApplication()->getInput()->request->getArray(), false, false);
	$event['result'] = $result;

But most of our events will be imutable and it will throw an error.
The patch allow to modyfy $event['result'] in ImutableEvent for now, with deprecation waring.

Testing Instructions

Code review.
Also can run:

$event = new class ('onFooBar') extends Joomla\CMS\Event\AbstractImmutableEvent{};
$event['result'] = 'foobar';

Actual result BEFORE applying this Pull Request

An error

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: Event b/c notes for 4.4 Manual#170
• No documentation changes for manual.joomla.org needed

[HLeithner]

@laoneo @MacJoom I think we need this before beta1