Fix deprecated warning for File.exists?

[johan-smits]

Since Ruby 3.0 this gives a warning.

[teksetter]

In ruby 3.2 it is a bug.
To solve the problem I modified the file "cupsffi/lib/cupsffi/printer.rb" line 133 fron

raise "File not found: #{file_name}" unless File.exists? file_name

to

raise "File not found: #{file_name}" unless File.exist? file_name

[spuder]

This is a issue on 3.3.6.

Work around is to monkey patch the File class or downgrade to ruby older than 3.2

class File
  def self.exists?(filename)
    self.exist?(filename)
  end
end