Note: the 'cjson-dev' dependency is required from 4.3.x,
while 4.3.x does not support Ruby 3.4.x out of the box.

Rather than work around supporting Ruby 3.4.x with 4.2.x,
just update it and add the dependency.

* Coroutines are using libucontext; added to ruby depends_dev.

* Asciidoctor updated to 2.x branch since we have newer Ruby now.

* uWSGI may not work properly with Rack; currently untested.

gettext-tiny 0.3.1 breaks on es/cs/it translations with a weird error.