Upgrade countries gem to 4.0.1

What does this MR do and why?

Upgrade countries gem to 4.0.1, which is ruby3 compliant.

Related to !106109 (closed) (this MR is a subset of those changes).

Gem diff:

• https://github.com/countries/countries/compare/v3.1.0...v4.0.0#diff-1db27d93186e46d3b441ece35801b244db8ee144ff1405ca27a163bfe878957f
• https://my.diffend.io/gems/countries/3.0.0/4.0.1

How to set up and validate locally

We use the World class in a couple of places:

• GitLab Subscriptions
• Country and CountryStates API 👉 !21417 (merged)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #383547 (closed)