Ignore plans and gitlab_subscriptions in cross modification check

Read more at https://gitlab.com/gitlab-org/gitlab/-/issues/343394 .

We are regularly seeing
CrossDatabaseModificationAcrossUnsupportedTablesError failures when we
call
[`Plan.default`](https://gitlab.com/gitlab-org/gitlab/-/blob/ad0085ce21be8d39b7598cb124adee7c75405922/app/models/plan.rb#L13)
and/or
[`Namespace#actual_plan`](https://gitlab.com/gitlab-org/gitlab/blob/master/ee/app/models/ee/namespace.rb#L180)
because both of these methods are lazily creating records.

At present we believe this is not strictly a problem that we need to
solve since it will is likely totally OK that creating a `plans` record
in the middle of a transaction related to `ci_pipelines` is not
important to rollback. Even more these lazy creations likely never need
to be rolled back anyway so we probably don't need to push these outside
the transactions.

spec/support/database/cross-database-modification-allowlist.yml

@@ -378,7 +378,6 @@
 - "./ee/spec/models/ci/build_spec.rb"
 - "./ee/spec/models/ci/minutes/additional_pack_spec.rb"
 - "./ee/spec/models/ci/pipeline_spec.rb"
-- "./ee/spec/models/ci/subscriptions/project_spec.rb"
 - "./ee/spec/models/concerns/approval_rule_like_spec.rb"
 - "./ee/spec/models/concerns/approver_migrate_hook_spec.rb"
 - "./ee/spec/models/dora/daily_metrics_spec.rb"

spec/support/database/prevent_cross_database_modification.rb

@@ -84,6 +84,11 @@ module Database
       parsed_query = PgQuery.parse(sql)
       tables = sql.downcase.include?(' for update') ? parsed_query.tables : parsed_query.dml_tables
 
+      # We have some code where plans and gitlab_subscriptions are lazily
+      # created and this causes lots of spec failures
+      # https://gitlab.com/gitlab-org/gitlab/-/issues/343394
+      tables -= %w[plans gitlab_subscriptions]
+
       return if tables.empty?
 
       cross_database_context[:modified_tables_by_db][database].merge(tables)