Update CI matrix for supported Ruby and Rails versions

[brendon]

This pull request updates the gem to support newer versions of Ruby and Rails, while dropping support for older versions. It also updates the development and CI environments to match these changes, including raising minimum dependency requirements and updating database service versions.

Dependency and compatibility updates:

• Raised minimum required Ruby version to 3.2 in unread.gemspec.
• Raised minimum required activerecord version to 7.2 in unread.gemspec.
• Updated Appraisals to add support for Rails 8.0 and 8.1, and removed appraisals for Rails 6.1, 7.0, and 7.1.
• Updated and renamed gemfiles to match new Rails versions and dependency requirements (gemfiles/rails_8.0.gemfile, gemfiles/rails_8.1.gemfile), and removed the Rails 6.1 gemfile [1] [2] [3].

Continuous Integration (CI) updates:

• Updated the CI matrix in .github/workflows/main.yml to test against Ruby 3.2, 3.3, 3.4, and 4.0, and Rails 7.2, 8.0, and 8.1, removing older versions [1] [2] [3].
• Updated the MySQL service in CI from version 5.7 to 8.0 to match modern environments.

[brendon]

I didn't realise you don't have GitHub actions enabled. If you turn them on the test suite should get a workout :)

[brendon]

Nice :)