Bump `mini_magick` to 4.12

What does this MR do and why?

This MR updates mini_magick gem to 4.12.

Changes: diffend

Based on the changelogs:

• Don't leave temporary files lying around when MiniMagick::Image#format failed
• Replace File.exists? with File.exist?, which should fix Ruby 3.2 compatibility
• Fixed a case where the log could not be parsed correctly when there were multiple lines
• Added status to the exception message when the ImageMagick command fails
• Allow passing format to Image#get_pixels so we can request "RGBA" pixels
• Suppress warning to ambiguous argument
• Use Thread#join with a timeout argument instead of the Timeout standard library

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.