Merge branch 'docs-add-note-about-backporting-for-add-columns-with-default' into 'master'

Add a note about backporting columns with default

See merge request gitlab-org/gitlab!32662

doc/development/migration_style_guide.md

@@ -559,6 +559,11 @@ Before PostgreSQL 11, adding a column with a default was problematic as it would
 have caused a full table rewrite. The corresponding helper `add_column_with_default`
 has been deprecated and will be removed in a later release.
 
+NOTE: **Note:**
+If a backport adding a column with a default value is needed for %12.9 or earlier versions,
+it should use `add_column_with_default` helper. If a [large table](https://gitlab.com/gitlab-org/gitlab/-/blob/master/rubocop/migration_helpers.rb#L12)
+is involved, backporting to %12.9 is contraindicated.
+
 ## Changing the column default
 
 One might think that changing a default column with `change_column_default` is an