docs: update README Ruby version support

[eitoball]

Summary

README.md still states that the gem requires Ruby 3.0 or later and is tested on Ruby 3.0 through 3.4. The CI matrix now covers Ruby 3.1 through 4.0.

Acceptance criteria

• Update the documented minimum Ruby version if Ruby 3.0 support has been dropped.
• Update the documented tested Ruby range to Ruby 3.1 through 4.0.
• Ensure the README accurately distinguishes runtime compatibility from CI coverage if they differ.