CI: refer to JRuby as "jruby" from setup-ruby, to get latest

[olleolleolle]

This PR introduces more JRuby versions to the build matrix.

[pirj]

I doubt we want to introduce new Ruby versions to the build.
For JRuby, there’s always the “are you using the latest?” question when there’s a bug. And we would really like to offload as our GHA gets overflown with two builds running 16+ jobs. Not to say JRuby jobs are usually the slowest to finish, so it may take 15+ minutes to a green build. Enough to steal focus and dry out the motivation to “fix a few minor things”.
Ideally, JRuby, TruffleRuby would run their builds against a stable version of RSpec, and we only keep builds for supported Ruby versions.
We’ve removed a bunch of Ruby versions in RSpec 4, but still 2.4-3.3 will all be there.

[JonRowe]

I'm a bit out of the loop on JRuby in terms of what the development cycle / actual usage is, versus strange bugs, we don't generally get a lot of JRuby specific bug reports these days giving me the tacit impression that its fairly stable. We keep some of the older versions around for legacy reasons, due to historic bugs and those will go away soon with RSpec 4 as @pirj mentions, I'm happy to keep a "current" version of JRuby in the CI suite but I'm not as interested in maintaining a spread of versions unless theres a good reason to?

[olleolleolle]

What I will do to this PR is keep a latest version, see how that feels.

I will not remove the legacy, ancient version's matrix elements, that's for a future change, I guess.

RSpec 4, yay for the future!