Change the getItem return type to stdClass and make the CMSObject inherit it

[laoneo]

Summary of Changes

To get away from the CMSObject class we need a step by step process in the getItem function of the admin model. Alternative to #42714 which makes the CMSObject a stdClass and changes the return type to it. It is a more lightweight solution as no new deprecated flag is introduced. It has basically the same effect as when we change the flag in 6.0.

Testing Instructions

Create an article in the back end. With as many content plugins as possible.

Actual result BEFORE applying this Pull Request

All works.

Expected result AFTER applying this Pull Request

All works.

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: getItem change for https://github.com/joomla/joomla-cms/pull/42717 Manual#232

• No documentation changes for manual.joomla.org needed

[crommie]

I have tested this item ✅ successfully on 080cca9

I've created an article with a number of custom fields and schema stuff. No difference before / after patch so I guess it works.

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[viocassel]

I have tested this item ✅ successfully on 080cca9

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[richard67]

RTC

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[LadySolveig]

Thank you @laoneo and also for testing @crommie and @viocassel