Upgrade to ruby 3.0.6 as minimum version; Preserve tabs by default

I've moved the minumum required version of Ruby to 3.0.6. I will drop support for ruby versions below 3.0. If you really want to use an unsupported version of Ruby, you can keep using paru version 1.1.x.

Also, in response to #79, paru now preserves tabs by default when running filters.