Fix deprecation warning on rubocop plugins/require change

[mjankowski]

Example deprecation from recent run: https://github.com/briandunn/flatware/actions/runs/20868195226/job/59964203799#step:4:7

[mjankowski]

This also:

• Adds minimum rubocop version, to avoid CI/local disconnects
• Regen appraisal files after recent changes (including that rubocop change)

[mjankowski]

Followup on this, in advance of what I assume might be an upcoming 2.4.0 release (to get ruby 4.0 support into tagged version?)... thoughts on any/all of the following as well:

• Ruby 3.1 is EOL and 3.2 will be in a few months, could drop 3.1 from CI matrix, and/or gemspec
• The rspec versions being excluded from CI matrix (for lack of ruby 4.0 support) are also pretty old ... could possibly bump the minimum version there at some point as it becomes less likely that anyone running a modern/supported ruby has also not updated rspec
• I'm not sure on full history here, but I noticed while doing this PR that rubocop-rspec is a) included in the gemfile, but b) commented out in rubocop config. There are ~90 RSpec/* offenses, with only ~30 autocorrectable. Might make sense to either remove that from gemfile and config, or fix them?

Let me know any thoughts on that stuff and if PRs would be welcome - otherwise will sit tight on tagged release.