Add Ruby 3.0 support

[ClearlyClaire]

• monkey-patch Terrapin and Cocaine to work around an issue with posix-spawn executing the child process with non-blocking file descriptors for stdout and stderr when using Ruby 3.0 (also fixed another issue not related to Ruby 3.0)
• monkey-patch Paperclip to fix usage of keyword arguments (in the future, looking at https://github.com/kreeti/kt-paperclip may be a better choice, but we currently depend on behavior from 6.0.0 that is changed in more recent releases)
• change keyword argument handling here and there

Actually running it on Ruby 3.0 is still not possible after that PR because one dependency (microformats) does not advertise Ruby 3.0 compatibility.

[ClearlyClaire]

Reading on the IO.select warning from DeepSource:

IO.select statements can get very hard to make them compatible with Ruby 3 scheduler. It is recommended to use wait_readable or wait_writable on an IO object with a given timeout instead of rescuing an IO::WaitReadable or IO::WaitWritable error and using IO.select in the block.

I think it's a bit early to really bother for now. As far as I understand, the idea is that Ruby's scheduler will eventually be able to schedule fibers, automatically yielding control to another fiber whenever hitting a supported IO operation, which read and wait_readable may be. However, I don't think this is fully implemented yet, and we do not make explicit use of fibers anyway.

I assume a good replacement to the pattern I used would eventually be to use fibers ourselves and have Ruby deal with them, but for now, I think it is fine using IO.select.

[tyteen4a03]

Late to the party, but a fresh install of 3.4.1 with a modified .ruby_version of 3.0.2 doesn't work out of the box.

I had to add rexml Gem (which apparently is no longer shipped by default in 3.0)