Add option to preserve old image when updating

[Woeler]

This is a simple change to allow preserving the old image file when the image is updating. I have a usecase where I do not want the original file to be deleted from the server. This simple optional setting allows for this.

simple usage:

AdvancedImage::make('image')
    ->disk('public')
    ->path('news')
    ->preserveOldFileOnUpdate(true),

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