Having `rack` gem is not a guarantee for having `rackup` executable (since Rack 3)

[dentarg]

Spotted this when creating #1355

heroku-buildpack-ruby/lib/language_pack/rack.rb

Lines 23 to 32 in 104fe3a

	def default_process_types
	# let's special case thin here if we detect it
	web_process = bundler.has_gem?("thin") ?
	"bundle exec thin start -R config.ru -e $RACK_ENV -p ${PORT:-5000}" :
	"bundle exec rackup config.ru -p ${PORT:-5000}"
	super.merge({
	"web" => web_process
	})
	end

Since Rack 3, that code needs another guard on the rackup gem: https://github.com/rack/rack/blob/v3.0.3/UPGRADE-GUIDE.md#binrackup-was-moved-to-a-separate-gem

[schneems]

Good catch. What do you think would be a good default here when there is no rackup gem? Specify no web default?

[dentarg]

Sounds resonable