Ignore plans and gitlab_subscriptions in cross modification check

What does this MR do and why?

Read more at #343394 (closed) .

We are regularly seeing CrossDatabaseModificationAcrossUnsupportedTablesError failures when we call Plan.default and/or Namespace#actual_plan 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.

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Related to #343394 (closed)