Fix crash with Ruby-3.2

[sudipm-mukherjee]

svn2git fails to work with Ruby-3.2 and gives the following error:

/usr/lib/ruby/vendor_ruby/svn2git/migration.rb:58:in `parse': undefined method `exists?' for File:Class (NoMethodError)

      if File.exists?(File.expand_path(DEFAULT_AUTHORS_FILE))
             ^^^^^^^^
Did you mean?  exist?
	from /usr/lib/ruby/vendor_ruby/svn2git/migration.rb:14:in `initialize'
	from /usr/bin/svn2git:26:in `new'
	from /usr/bin/svn2git:26:in `<main>'