Add deprecation message for user aid property

[laoneo]

Followup of #39620 to deprecate the property.

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: User $aid deprecation Manual#88

• No documentation changes for manual.joomla.org needed

[joomdonation]

I have tested this item ✅ successfully on 7574b68

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[alikon]

I have tested this item ✅ successfully on 7574b68

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[alikon]

RTC

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