[12-1-stable] Re-add ignore_column for import columns

Stan Hurequested to merge
sh-fix-no-downtime-upgrades-ce into 12-1-stable-patch-10
  1. !29863 (merged) dropped the import columns and the ignore_columns.
  2. !29959 (merged) readded ignore_columns to avoid no-downtime upgrade issues in 12.1.
  3. !30725 (merged) removed these ignore_columns entries again.

We need to keep these in 12.1 to ensure no-downtime upgrades continue to work: gitlab#32180 (comment 218556216)

Edited by Stan Hu

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