Update default Rails version to the latest of released version

Using released latest is a good way to develop gem with other gems.

Author: tricknotes

Gemfile

@@ -2,7 +2,7 @@ source "https://rubygems.org"
 
 gemspec
 
-rails_version = ENV.fetch("RAILS_VERSION", "6.0")
+rails_version = ENV.fetch("RAILS_VERSION", "7.2")
 
 if rails_version == "main"
   rails_constraint = { github: "rails/rails" }