DEV: Add a HasDeprecatedColumns concern for better deprecation messages (#22930)

Currently when we decide we're going to drop a column in the future we just mark it with a TODO comment and add it to ignored_columns. This makes it instantly unavailable, and we mostly forget about the TODO in the end. 😬

This change adds a HasDeprecatedColumns concern which offers a little bit more flexibility. We can still simulate the old behaviour by setting drop_from to the current version, but we can also set it to a future version, causing it to raise a deprecation warning until then if used.

app/models/concerns/has_deprecated_columns.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+module HasDeprecatedColumns
+  extend ActiveSupport::Concern
+
+  class_methods do
+    def deprecate_column(column_name, drop_from:, raise_error: false, message: nil)
+      if Gem::Version.new(Discourse::VERSION::STRING) >= Gem::Version.new(drop_from)
+        self.ignored_columns = self.ignored_columns.dup << column_name.to_s
+      else
+        message = message.presence || "column `#{column_name}` is deprecated"
+
+        define_method(column_name) do
+          Discourse.deprecate(message, drop_from: drop_from, raise_error: raise_error)
+          super()
+        end
+
+        define_method("#{column_name}=") do |value|
+          Discourse.deprecate(message, drop_from: drop_from, raise_error: raise_error)
+          super(value)
+        end
+      end
+    end
+  end
+end