Update ruby and webricks to latest

[IAlibay]

I believe that both ruby 3.x and webricks 1.8 are now supported by github pages so we should be able to upgrade.

[orbeckst]

I don't know what the docker image (see PR #334 ) uses internally. From my experience with ruby/gems/whatever, I am happy if it builds at all. I suppose we could just try it and see if the website still builds.

[RMeli]

Related to #380?