Drop support for Ruby 3.0

[sarahraqueld]

Ruby 3.0 reached EOL on April 2024, but we're still running that version on CI:

Editing the workflow build file to remove 3.0 should be enough to close this issue: https://github.com/thoughtbot/factory_bot/blob/main/.github/workflows/build.yml

[CodeMeister]

If Ruby 3.0 is working with Rails 7.0 and 7.1 then it looks like it's the gem resolution with Rails 6.1 is the issue.

I'm hesitant for 3.0 to be removed as there are still smaller companies using this version. (yes, they should really upgrade, but many don't have the budget available)

A simpler resolution to stop the automated tests failing would be to exclude Rails 6.1 when testing Ruby 3.0.

However, if FactoryBot now needs gems that don't run on 3.0 then I would suggest bumping the major version number, so Ruby 3.0 developers will know not jump versions.