Convert 20241022181600 to a schema migration

This migration caused the BBM to begin erroring on .com because
application code is updated _before_ post-deployment migrations run. For
.com the issue was addressed by pausing the migration until the
post-deployment migrations were able to run an delete it. This change
converts the migration to a schema migration so that the same issue does
not happen during self-managed upgrades.

Changelog: fixed

db/post_migrate/20241022181600_remove_backfill_vulnerability_occurrence_pipelines_project_id.rb → db/migrate/20241022181600_remove_backfill_vulnerability_occurrence_pipelines_project_id.rb

@@ -7,6 +7,8 @@ class RemoveBackfillVulnerabilityOccurrencePipelinesProjectId < Gitlab::Database
   MIGRATION = "BackfillVulnerabilityOccurrencePipelinesProjectId"
 
   def up
+    # rubocop:disable Migration/BatchMigrationsPostOnly -- Must be run before BBM code is deleted.
+    # Estimated runtime on .com is only 5 seconds.
     delete_batched_background_migration(
       MIGRATION,
       :vulnerability_occurrence_pipelines,
@@ -18,6 +20,7 @@ def up
         :occurrence_id
       ]
     )
+    # rubocop:enable Migration/BatchMigrationsPostOnly
   end
 
   def down