Update CI Configuration for IRuby

[kojix2]

• Deleted Ruby 2.5 and 2.6 from the CI matrix
• Removed IRB installation logic for older Ruby versions
• Added ffi 1.6 installation for Ruby 2.7
• Update checkout action to v4
• Use the latest ruby version

[kojix2]

If you are using Ruby 2.6 or earlier and want IRuby's CI to work on Ruby 2.6, please send a pull request.