Follow-up from "Update scheduled_scans_max_concurrency for self-managed"

The following discussion from !157828 (merged) should be addressed:

• @project_278964_bot_445da1b60fc7336b6b6776383134d10f started a discussion: (+1 comment)

  Changing column default is difficult because of how Rails handles values that are equal to the default. Please make sure all columns are declared as columns_changing_default. For more information, see Avoiding downtime in migrations documentation.