Vendor Foundation form builder to remove gem dependency

The "foundation_rails_helper" gem is no longer maintained and is
incompatible with Rails 7.1. To avoid blocking the upgrade, we've vendored
the vendor/foundation_rails_helper/form_builder.rb as a copy of the
original FormBuilder class.

To mantain compatibility with auto_labels and button_class variables, that
are used in the original builder, we are overwriting in the foundation
form builder initializer.

The gem has been removed from the Gemfile and replaced with this vendored
fallback. This workaround is safe to remove once legacy Foundation CSS
support is dropped.

All vendored code retains the original MIT license and attribution.

config/initializers/foundation_form_builder.rb

@@ -0,0 +1,16 @@
+require Rails.root.join("vendor/foundation_rails_helper/form_builder.rb")
+
+class FoundationRailsHelper::FormBuilder
+  def column_classes(...)
+    ""
+  end
+
+  def auto_labels
+    true
+  end
+
+  def submit(value = nil, options = {})
+    options[:class] ||= "success button"
+    super
+  end
+end