Add error handler warning to update guide

[dabrt]

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Add error handler warning to update guide

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[reithor]

@dabrt current suggestion is technically correct imho.
Alternatively we could add the information just before https://doc.ibexa.co/en/latest/update_and_migration/from_4.6/update_from_4.6/#v461 :

--
To avoid deprecations when using PHP 8.2 or PHP 8.3, v4.6 provides an additional error handler.
If the error handler is not already present in composer.json, add the error handler by running the following commands:

composer config extra.runtime.error_handler "\\Ibexa\\Contracts\\Core\\MVC\\Symfony\\ErrorHandler\\Php82HideDeprecationsErrorHandler"
composer dump-autoload

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[mnocon]

Thank you!

[adriendupuis]

Not sure this caution box will be well understood when on 5.0 doc https://doc.ibexa.co/en/5.0/infrastructure_and_maintenance/performance/

[adriendupuis]

It's clear that "update_from_4.6" is only about 4.6 and in the continuation "update_to_5.0" removes it.

But for "performance" when on 5.0 branch of our doc, this is less clear. It's not since 4.6 that this is needed, it's for 4.6 only. I would add something about 5.0 not needing this, or I would have it only on 4.6 branch but not on 5.0 branch.