Remove Ruby 2.5 support

[mattwynne]

Summary

Remove support for Ruby 2.5

Motivation and Context

It's no longer supported. It's breaking the build.

Types of changes

• [x] Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist:

• [ ] I've added tests for my code.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.