Support ActiveRecord 8.0

[akouryy]

Fixes #20.

This pull request adds support for ActiveRecord 8.0.
I have run the examples in the README against rails new locally to confirm that they work as expected.

In this PR, I chose to remove the upper version limit on the dependency, primarily to reduce the maintenance burden.

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• gemfiles/ar80.gemfile: Language not supported

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armg.gemspec:26

• There is a discrepancy between the gemspec dependency (>= 7) and the changelog which mentions support for ActiveRecord 7.1 and 7.2. Please verify whether ActiveRecord 7.0 is fully supported or update the documentation and changelog to reflect the correct minimal supported version.

spec.add_dependency 'activerecord', '>= 7'

.github/workflows/test.yml:30

• The inline comment in the exclusion list may be interpreted as an array element. Consider removing or relocating the comment to ensure the exclusion block functions as intended.

- # Ruby 3.1 is not supported by ActiveRecord 8.0

[akouryy]

I just discovered that the files under gemfiles/ are automatically generated by Appraisal. So, I've added a commit to update the Appraisals file to reflect the changes in this PR and #21.

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• gemfiles/ar70.gemfile: Language not supported
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