Ruby 3 incompatibility (call to `File.exists?`)

[hakanai]

cane checks currently fail on Ruby 3 because of this call to File.exists?:

https://github.com/square/cane/blob/c8d6ce4e0092e5f677d3a2f409db7566e5d82136/lib/cane/file.rb#L21-L23

This now gives a NoMethodError and suggests File.exist?.

Typical Ruby to delete the method with the correct grammar, if you ask me. :(