[9.x] Remove extra code in Model.php to optimize performance 🏃🏻‍♂️

[imanghafoori1]

First Commit:

• The whereKey method can accept an array of ids so the check is not really needed.
• It also kinda optimizes since the whereKey method now uses the whreIntegerInRaw (instead of whereIn) for numeric ids.

Second commit:

• The second commit converts anonymous function to arrow functions.