Run migrations in reverse order in CI check

In the CI job that validates migration schema changes, the current
behavior did not account for multiple migrations in one MR, and so ran
the migrations based on the output of `git diff`.

This change fixes that, so all migrations in `post_migrate` will run
first in reverse timestamp order, and then migrations in `migrate` will
run in reverse timestamp order.

scripts/validate_migration_schema

@@ -27,7 +27,7 @@ class MigrationSchemaValidator
   private
 
   def validate_schema_on_rollback!
-    committed_migrations.each do |filename|
+    committed_migrations.reverse_each do |filename|
       version = find_migration_version(filename)
 
       run("bin/rails db:migrate:down VERSION=#{version}")