Use generic Rackup::Handler.default

[voxik]

Move from specific Rack::Handler::WEBrick to more generic Rackup::Handler.default. This chooses the best available handler automatically. If more handlers are available, RACKUP_HANDLER environment variable could be used to override the default.

ATM, Gemfile still specifies WEBrick, so this is the one which will be used. But replacing WEBrick by Puma should be as easy as updating the Gemfile

This is followup to #244 and this idea comes from this discussion

[felipedmesquita]

Hi, @voxik. Thanks for your continued work on this!
After your rack 3 upgrade, I've merged a couple small ci fixes (removed ruby 2.5, added 3.3 and 3.4, skipped a failing test on mac due to a libcurl bug) and now we are only failing checks for ruby 2.6.
I have a branch (felipedmesquita#11) with some ugly defined? conditionals that passes all checks. I would appreciate your thoughts on if that is the right approach or there is a cleaner way so we get everything green. Also, could you please rebase this on the latest master so we get only the 2.6 failing checks?

[voxik]

This got broken by #255. Now the question is how to proceed. I'll likely update the PR so the "Rackup" branches use the generic Rackup::Handler.default and leaving the "Rack" branches to use the legacy mechanism? Any thoughts?