Add link to issue about schema_migration versions

Add a comment in structure.sql that links to the announcement issue for
the changes to how schema_migration versions are handled.

db/structure.sql

@@ -22919,3 +22919,4 @@ ALTER TABLE public.product_analytics_events_experimental
 
 -- schema_migrations.version information is no longer stored in this file,
 -- but instead tracked in the db/schema_migrations directory
+-- see https://gitlab.com/gitlab-org/gitlab/-/issues/218590 for details

lib/gitlab/database/schema_cleaner.rb

@@ -26,6 +26,7 @@ module Gitlab
         io << <<~MSG
           -- schema_migrations.version information is no longer stored in this file,
           -- but instead tracked in the db/schema_migrations directory
+          -- see https://gitlab.com/gitlab-org/gitlab/-/issues/218590 for details
         MSG
 
         nil

spec/fixtures/gitlab/database/structure_example_cleaned.sql

@@ -29,3 +29,4 @@ CREATE INDEX index_abuse_reports_on_user_id ON public.abuse_reports USING btree
 
 -- schema_migrations.version information is no longer stored in this file,
 -- but instead tracked in the db/schema_migrations directory
+-- see https://gitlab.com/gitlab-org/gitlab/-/issues/218590 for details