Test ruby 3.1 & 3.2, drop 2.5 & 2.6

[jdelStrother]

Which version you want to support is up to you, but given ruby 2.6 is no longer supported by the ruby team, maybe it's time to drop it from CI?

The matrix.exclude option is getting kind of verbose - wondered if there was a shorter alternative with matrix.include but not sure it's possible.

[jdelStrother]

I see I missed bumping the manticore service. I'll take another run at this tomorrow

[pat]

Thanks @jdelStrother for all the patience and prodding. I'm going to merge this in now, happy for Manticore update to be separate. Plus, I want to see if I can wrangle better pagination support for Manticore, but that'll be after the upcoming release.

[pat]

Also worth noting: there's still CircleCI tests that run against Rails 4.2 and Sphinx 2.2 (using Ruby 2.4 😅), because I can't install Sphinx 2.2 on GitHub's more 'modern' workers (Ubuntu 18.04). But maybe it's well and truly time to retire Rails 4.2 support! It's more the loss of testing against Sphinx 2.2.11 that I'm not thrilled about.