Drop support for older rubies, rails and redis

[fatkodima]

Also a candidate for a new major release, imo.

This PR drops support for:

1. ruby < 3.1 (https://devcenter.heroku.com/articles/ruby-support-reference#supported-runtimes, https://endoflife.date/ruby)
2. rails < 7
3. redis < 6 (https://redis.io/docs/latest/operate/rs/installing-upgrading/product-lifecycle/).

It also updates CI to check against newer railses.

There is a mention about jruby in the readme, but no tests on CI. I would suggest just dropping it, as I am not sure many people (anyone?) uses it in production to worth the effort.

[fatkodima]

I also noticed, that there is a rubocop and standardrb. I would suggest removing the latter and properly use rubocop and fix all the current offenses.

Also, there is a

coverband/Rakefile

Line 11 in 204d2d8

task "test:all": %i[rubocop test forked_tests benchmarks:memory benchmarks]

task, which runs forked_tests. These are not ran on CI currently and are broken. That should be also fixed.

I can help, if ok.