Support activerecord 8.1 explicitly

[ohbarye]

Change

This adds activerecord 8.1 into the test matrix since activerecord 8.1 was released on October 22, 2025.

ref: https://rubygems.org/gems/activerecord/versions/8.1.0

8.1 still requires Ruby 3.2 or later as like 8.0 does.

Testing

Some tests fail on 8.1. I'll check it later. https://github.com/ohbarye/activerecord-debug_errors/actions/runs/19865227719/job/56925996094