Replace customisations with explicit patch in an initializer

See previous commit.

This isn't nice, but it does the same job as the gem did. We can also update this code now as Rails changes (the gem was incompatible with Rails 7).

config/initializers/custom_errors.rb

@@ -0,0 +1,22 @@
+# This patch customises ActiveModel error messages, as previously handled by the custom_error_messages gem
+# See: https://github.com/jeremydurham/custom-err-msg
+
+module ActiveModel
+  class Error
+    def self.full_message(attribute, message, base)
+      return message if attribute == :base
+
+      attr_name = attribute.to_s.tr(".", "_").humanize
+      attr_name = base.class.human_attribute_name(attribute, {
+        default: attr_name,
+        base: base,
+      })
+
+      if message.start_with?("^")
+        I18n.t("errors.format.full_message", default: "%{message}", message: message[1..-1], attribute: attr_name)
+      else
+        I18n.t("errors.format", default: "%{attribute} %{message}", message: message, attribute: attr_name)
+      end
+    end
+  end
+end