Changes "define" to "declare" in the deprecation message in `WP_List_Table` magic methods.
Why is "declare" better?
It aligns well to:
* the topic of and published information about dynamic properties.
* the act of explicitly listing the variable as a property on the class.
The goal of this message is guide developers to change their code. Changing the term to "declare" hopefully will aid in the understanding of what is being asked of developers when this deprecation is thrown.
Follow-up [56349].
Props hellofromTonya, antonvlasenko.
Fixes#58896.
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