Proposal: ditch EOL'd Ruby versions and release a major version

This Issue is a proposal: to ease maintenance of this gem, keep only Ruby and Rails versions that are currently in support, in the CI matrix and the required_ruby_version and the Rails dependencies.

Currently:

- Ruby 3.2+
- Rails 7.2 is in active support (for 2 more weeks!)

Start by locating the CI matrix, required_ruby_version, and Rails dependency declarations mentioned in the proposal. Check the currently supported Ruby and Rails versions against those settings, then review the release configuration or documentation to determine what a major-version release requires. Done means the supported-version policy is applied consistently and the major release is prepared.

Written by the indexing model from the issue text.