Default gemfile generator to puma >= 6.0

Puma 5.x is not compatible with Rack 3 cookies. The current default
new Gemfile uses puma ">= 5.0". Puma ~> 5.6 will error
on start when used with Rack 3, but earlier versions will not.

puma/puma#3164: Support Rack 3 cookies
puma/puma#3166: Prevent loading with rack 3
phusion/passenger#2503: Related passenger ex

railties/lib/rails/generators/app_base.rb

@@ -272,7 +272,7 @@ def database_gemfile_entry # :doc:
       end
 
       def web_server_gemfile_entry # :doc:
-        GemfileEntry.new "puma", ">= 5.0", "Use the Puma web server [https://github.com/puma/puma]"
+        GemfileEntry.new "puma", ">= 6.0", "Use the Puma web server [https://github.com/puma/puma]"
       end
 
       def asset_pipeline_gemfile_entry