Remove 6.1 7.0

[lcreid]

Rails 6.1 has been off support for some time. Rails 7.0 is about to go off support. Both are causing challenges to update the Selenium versions for running system tests locally (#745). It's nice to keep testing old versions, but when it gets in the way of other work, I'm going to be realistic and stop banging my head against a wall to support old versions.

Other changes:

• Removes Ruby 3.0 and 3.1, and adds Ruby 3.4.
• Removes pry-byebug because it's old and will soon start causing conflicts (it did in Upgrade selenium #745).
• ostruct isn't in the standard library soon, so add it for Ruby versions that don't have it (or give deprecation messages).
• Bumps the RuboCop checks to the minimum supported (Ruby 3.2 and Rails 7.1). This required some autocorrect for anonymous blocks, which I'm not particularly fond of, but I'll go with the community.
• Removes adding of a commit for changes to the screenshots. The screenshot generation is broken, probably due to version incompatibility. That should be fixed in Upgrade selenium #745, but in the meantime there was no way to merge this PR (and in fact, there never was a way to actually ignore screenshot changes, because they would always be regenerated).