Adjust CI Matrix Ruby Version to 3.0+ to align with gemspec

[taketo1113]

Summary

This PR addresses inconsistencies between the Ruby versions specified in the gemspec and the CI matrix.
In paper_trail v.16.0.0, the gemspec requires Ruby 3.0 or higher, while the CI matrix specifies Ruby 3.1 or higher.

The CI matrix was updated to require Ruby 3.1+ in the PR (#1485), where Ruby 3.0 was temporarily removed to support Rails 7.2

Decide what to do with Ruby 3.0 tests. Rails 7.2.0 does not support this version, so you should completely change test workflow. I temporarily removed it.

Details

This PR changes the Ruby version in the CI matrix to 3.0 or higher to align with the required_ruby_version in the gemspec.
Additionally, the Compatibility section in the README has been updated to match the gemspec.

Additional information

If support for Ruby 3.0 is dropped, I think it will also be necessary to update the gemspec and RuboCop versions.
https://github.com/paper-trail-gem/paper_trail/blob/master/.github/CONTRIBUTING.md#lowest-supported-ruby-version

Check the following boxes:

• Wrote good commit messages.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new
  code introduces user-observable changes.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.