deprecated methods no longer existing on new ruby versions

[dann1]

Description
Some deprecated methods like File.exists? are still being used in ruby libraries. This method no longer exists on newer ruby versions and therefore throws errors, which will result in eventual incompatibility.

To Reproduce

• Install the opennebula-cli gem on a client with a recent ruby version

ruby 3.2.2 (2023-03-30 revision e51014f9c0) [x86_64-darwin22]

• issue the onevm list command

 ~  onevm list                                                                                                                                           Wed Jun 21 11:23:45 2023
undefined method `exists?' for File:Class

Expected behavior
The commands should work with recent ruby versions.

Additional context

• This can easily be solved with a massive linting as rubocop will point out the deprecations
• The rubocop exclusions should be removed to prevent these problems. Every file should be linted.
• The CLI client is MacOS with ruby installed from Homebrew

Progress Status

• Code committed
• Testing - QA
• Documentation (Release notes - resolved issues, compatibility, known issues)