Add IPv6 support (#165)

Short: The industry is moving from IPv4 and IPv6 is the future. Here's some more context heroku/roadmap#40.

Internal Heroku link for more context: https://salesforce-internal.slack.com/archives/C06NM9FFY57/p1732035124610499?thread_ts=1731970807.940289&cid=C06NM9FFY57

config/puma.rb

@@ -4,7 +4,8 @@
 
 preload_app!
 
-port        ENV.fetch("PORT") { 3000 }
+# Support IPv6 by binding to host `::` instead of `0.0.0.0`
+port(ENV.fetch("PORT") { 3000 }, "::")
 environment ENV.fetch("RAILS_ENV") { "development" }
 
 # Allow puma to be restarted by `rails restart` command.