Add support for activerecord 8.0

[tnir]

This allows us to use activerecord 8.0.

In addition,

• Adds Ruby 3.3 and head (3.4 as of writing) to all test matrices (only with Rails 7.0, 7.1, 8.0, and edge (i.e., 8.1))
• Adds Ruby 3.2 to MySQL test matrix
• Tweaks mutex_m / bigdecimal gems only in Rails 7.0.x only on testing
• Allows us to use rspec-rails 7.x, which is officially for Rails 7.0+

Checklist

• Test on my forked repo in advance of approving worlflow: https://github.com/tnir/awesome_nested_set/actions/runs/11425291192

[parndt]

Thank you @tnir ! 🎉