PMPro_Discount_Code class attributes created dynamically throw deprec…

[MaximilianoRicoTabo]

…ated warnings in PHP 8.2

Declare dynamic created props as class attributes.

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[dparker1005]

Looks good! Though I noticed setting the $code property is missing in get_discount_code_by_code(). Can we declare the $code property in this PR and squeeze setting it into that get_discount_code_by_code() function?