[4.2] PHP8.2 Define the folder property in the media form field

[laoneo]

Summary of Changes

Define the folder property in the MediaField class to prevent deprecation notice on PHP 8.2.

Testing Instructions

Open the article form on the PHP 8.2 on the back end with debug enabled.

Actual result BEFORE applying this Pull Request

The following warning is shown:
Creation of dynamic property Joomla\CMS\Form\Field\MediaField::$folder is deprecated in

Expected result AFTER applying this Pull Request

No warning.

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:

• No documentation changes for manual.joomla.org needed

[joomdonation]

I have tested this item ✅ successfully on afeada2

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/39602.}

[dgrammatiko]

I have tested this item ✅ successfully on afeada2

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/39602.}

[Quy]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/39602.}

[fancyFranci]

Great to "see" another vanishing warning. Thank you!