[12-1-stable-ee] Fix no-downtime upgrades

1. gitlab-foss!29863 (merged) dropped the import columns and the ignore_columns.
2. gitlab-foss!29959 (merged) readded ignore_columns to avoid no-downtime upgrade issues in 12.1.
3. gitlab-foss!30725 (merged) removed these ignore_columns entries again.

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

CE port: gitlab-foss!32977 (merged)

Once merged and backported

• Remove omnibus-gitlab!3612 (merged)