Support ruby 3

[SuperTux88]

I already fixed some issues to support ruby 3 in #8368, but there are still some issues that don't work yet with ruby 3.0.x:

• [ ] i18n-inflector-rails isn't compatible with ruby 3, and the last release is from 2013. But there has been some activity in the Repo, and it looks like the issue with ruby 3 was fixed ... but there hasn't been a release yet. And the fix also changed the requirement to be ruby >= 3, so we can't support ruby 2.7 at the same time (which means as soon as we would ruby 3 support with it, we need to drop 2.7, so we can't have a release that supports 2.7 AND 3). There are multiple options now:
  • Wait until a new version is released, update to that, and drop ruby 2.7
  • Find a different solution to achieve the same, which maybe already works with ruby 3 (and also 2.7)
  • Drop the dependency, as far as I can tell it's only used in 8 translation strings (mostly in pl)
• [ ] there is a problem with carrierwave and mini_magick. Basically the feature added in #7924 doesn't work in ruby 3: convert: unrecognized gravity type `{:combine_options=>{:unsharp=>"1.5x1+0.7+0.02"}}' @ error/convert.c/ConvertImageCommand/1762., because it adds the combine_options to the gravity parameter. As far as I can tell there isn't a fix for that yet.
• [ ] With ruby 3.1 the tests don't even start, and I haven't looked closer into that. Lets first focus on getting ruby 3.0 in a working state.